diff --git a/src/type/definition.js b/src/type/definition.js
index 17c3c16e87..e78fa9a81a 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -12,6 +12,7 @@ import invariant from '../jsutils/invariant';
 import isNullish from '../jsutils/isNullish';
 import * as Kind from '../language/kinds';
 import { assertValidName } from '../utilities/assertValidName';
+import { scalarValueFromAST } from '../utilities/scalarValueFromAST';
 import type {
   ScalarTypeDefinitionNode,
   ObjectTypeDefinitionNode,
@@ -345,7 +346,10 @@ export class GraphQLScalarType {
   // Parses an externally provided value to use as an input.
   parseValue(value: mixed): mixed {
     const parser = this._scalarConfig.parseValue;
-    return parser && !isNullish(value) ? parser(value) : undefined;
+    if (isNullish(value)) {
+      return undefined;
+    }
+    return parser ? parser(value) : value;
   }
 
   // Determines if an internal value is valid for this type.
@@ -357,7 +361,7 @@ export class GraphQLScalarType {
   // Parses an externally provided literal value to use as an input.
   parseLiteral(valueNode: ValueNode): mixed {
     const parser = this._scalarConfig.parseLiteral;
-    return parser ? parser(valueNode) : undefined;
+    return (parser || scalarValueFromAST)(valueNode);
   }
 
   toString(): string {
diff --git a/src/utilities/__tests__/buildASTSchema-test.js b/src/utilities/__tests__/buildASTSchema-test.js
index c92ed212d3..68c5f77cbe 100644
--- a/src/utilities/__tests__/buildASTSchema-test.js
+++ b/src/utilities/__tests__/buildASTSchema-test.js
@@ -444,6 +444,22 @@ describe('Schema Builder', () => {
     expect(output).to.equal(body);
   });
 
+  it('Custom scalar argument field with default', () => {
+    const body = dedent`
+      schema {
+        query: Hello
+      }
+
+      scalar CustomScalar
+
+      type Hello {
+        str(int: CustomScalar = 2): String
+      }
+    `;
+    const output = cycleOutput(body);
+    expect(output).to.equal(body);
+  });
+
   it('Simple type with mutation', () => {
     const body = dedent`
       schema {
diff --git a/src/utilities/__tests__/buildClientSchema-test.js b/src/utilities/__tests__/buildClientSchema-test.js
index 49919f8599..1e891fb736 100644
--- a/src/utilities/__tests__/buildClientSchema-test.js
+++ b/src/utilities/__tests__/buildClientSchema-test.js
@@ -343,6 +343,30 @@ describe('Type System: build schema from introspection', () => {
     await testSchema(schema);
   });
 
+  it('builds a schema with default value on custom scalar field', async () => {
+    const schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'ArgFields',
+        fields: {
+          testField: {
+            type: GraphQLString,
+            args: {
+              testArg: {
+                type: new GraphQLScalarType({
+                  name: 'CustomScalar',
+                  serialize: value => value
+                }),
+                defaultValue: 'default'
+              }
+            }
+          }
+        }
+      })
+    });
+
+    await testSchema(schema);
+  });
+
   it('builds a schema with an enum', async () => {
     const foodEnum = new GraphQLEnumType({
       name: 'Food',
diff --git a/src/utilities/__tests__/scalarValueFromAST-test.js b/src/utilities/__tests__/scalarValueFromAST-test.js
new file mode 100644
index 0000000000..01f39b1c1b
--- /dev/null
+++ b/src/utilities/__tests__/scalarValueFromAST-test.js
@@ -0,0 +1,77 @@
+/**
+ *  Copyright (c) 2017, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import { scalarValueFromAST } from '../scalarValueFromAST';
+import { parseValue } from '../../language';
+
+describe('scalarValueFromAST', () => {
+
+  function testCase(valueText, expected) {
+    expect(
+      scalarValueFromAST(parseValue(valueText))
+    ).to.deep.equal(expected);
+  }
+
+  function testNegativeCase(valueText, errorMsg) {
+    expect(
+      () => scalarValueFromAST(parseValue(valueText))
+    ).to.throw(errorMsg);
+  }
+
+  it('parses simple values', () => {
+    testCase('null', null);
+    testCase('true', true);
+    testCase('false', false);
+    testCase('123', 123);
+    testCase('123.456', 123.456);
+    testCase('"abc123"', 'abc123');
+  });
+
+  it('parses lists of values', () => {
+    testCase('[true, false]', [ true, false ]);
+    testCase('[true, 123.45]', [ true, 123.45 ]);
+    testCase('[true, null]', [ true, null ]);
+    testCase('[true, ["foo", 1.2]]', [ true, [ 'foo', 1.2 ] ]);
+  });
+
+  it('parses input objects', () => {
+    testCase(
+      '{ int: 123, requiredBool: false }',
+      { int: 123, requiredBool: false }
+    );
+    testCase(
+      '{ foo: [{ bar: "baz"}]}',
+      { foo: [ { bar: 'baz'} ] }
+    );
+  });
+
+  it('rejects enum values and query variables', () => {
+    testNegativeCase('TEST_ENUM_VALUE', 'Scalar value can not contain Enum.');
+    testNegativeCase(
+      '$test_variable',
+      'Scalar value can not contain Query variable.'
+    );
+    testNegativeCase('[TEST_ENUM_VALUE]', 'Scalar value can not contain Enum.');
+    testNegativeCase(
+      '[$test_variable]',
+      'Scalar value can not contain Query variable.'
+    );
+    testNegativeCase(
+      '{foo: TEST_ENUM_VALUE}',
+      'Scalar value can not contain Enum.'
+    );
+    testNegativeCase(
+      '{bar: $test_variable}',
+      'Scalar value can not contain Query variable.'
+    );
+  });
+
+});
diff --git a/src/utilities/buildASTSchema.js b/src/utilities/buildASTSchema.js
index f3d0e6dac2..9d05423a6c 100644
--- a/src/utilities/buildASTSchema.js
+++ b/src/utilities/buildASTSchema.js
@@ -447,13 +447,7 @@ export function buildASTSchema(ast: DocumentNode): GraphQLSchema {
       name: def.name.value,
       description: getDescription(def),
       astNode: def,
-      serialize: () => null,
-      // Note: validation calls the parse functions to determine if a
-      // literal value is correct. Returning null would cause use of custom
-      // scalars to always fail validation. Returning false causes them to
-      // always pass validation.
-      parseValue: () => false,
-      parseLiteral: () => false,
+      serialize: value => value,
     });
   }
 
diff --git a/src/utilities/buildClientSchema.js b/src/utilities/buildClientSchema.js
index fca78567ed..ba11fdd3e4 100644
--- a/src/utilities/buildClientSchema.js
+++ b/src/utilities/buildClientSchema.js
@@ -227,12 +227,6 @@ export function buildClientSchema(
       name: scalarIntrospection.name,
       description: scalarIntrospection.description,
       serialize: id => id,
-      // Note: validation calls the parse functions to determine if a
-      // literal value is correct. Returning null would cause use of custom
-      // scalars to always fail validation. Returning false causes them to
-      // always pass validation.
-      parseValue: () => false,
-      parseLiteral: () => false,
     });
   }
 
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 0c3b76c9a4..08d776190e 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -499,12 +499,6 @@ export function extendSchema(
       description: getDescription(typeNode),
       astNode: typeNode,
       serialize: id => id,
-      // Note: validation calls the parse functions to determine if a
-      // literal value is correct. Returning null would cause use of custom
-      // scalars to always fail validation. Returning false causes them to
-      // always pass validation.
-      parseValue: () => false,
-      parseLiteral: () => false,
     });
   }
 
diff --git a/src/utilities/index.js b/src/utilities/index.js
index 65feccbd51..729c399a8d 100644
--- a/src/utilities/index.js
+++ b/src/utilities/index.js
@@ -58,6 +58,10 @@ export { valueFromAST } from './valueFromAST';
 // Create a GraphQL language AST from a JavaScript value.
 export { astFromValue } from './astFromValue';
 
+// Create a JavaScript value from a GraphQL language AST representation
+// of Scalar.
+export { scalarValueFromAST } from './scalarValueFromAST';
+
 // A helper to use within recursive-descent visitors which need to be aware of
 // the GraphQL type system.
 export { TypeInfo } from './TypeInfo';
diff --git a/src/utilities/scalarValueFromAST.js b/src/utilities/scalarValueFromAST.js
new file mode 100644
index 0000000000..bfb1f677fb
--- /dev/null
+++ b/src/utilities/scalarValueFromAST.js
@@ -0,0 +1,43 @@
+/* @flow */
+/**
+ *  Copyright (c) 2017, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+import keyValMap from '../jsutils/keyValMap';
+import * as Kind from '../language/kinds';
+import type { ValueNode } from '../language/ast';
+
+/**
+ * Create a JavaScript value from a GraphQL language AST representation
+ * of Scalar.
+ */
+export function scalarValueFromAST(astValue: ValueNode): mixed {
+  switch (astValue.kind) {
+    case Kind.NULL:
+      return null;
+    case Kind.INT:
+      return parseInt(astValue.value, 10);
+    case Kind.FLOAT:
+      return parseFloat(astValue.value);
+    case Kind.STRING:
+    case Kind.BOOLEAN:
+      return astValue.value;
+    case Kind.LIST:
+      return astValue.values.map(scalarValueFromAST);
+    case Kind.OBJECT:
+      return keyValMap(
+        astValue.fields,
+        field => field.name.value,
+        field => scalarValueFromAST(field.value),
+      );
+    case Kind.ENUM:
+      throw new Error('Scalar value can not contain Enum.');
+    case Kind.VARIABLE:
+      throw new Error('Scalar value can not contain Query variable.');
+  }
+}