diff --git a/src/auth/authclient.ts b/src/auth/authclient.ts
index ab10eac4..f7d4d13c 100644
--- a/src/auth/authclient.ts
+++ b/src/auth/authclient.ts
@@ -17,12 +17,14 @@ import {GaxiosOptions, GaxiosPromise} from 'gaxios';
 
 import {DefaultTransporter} from '../transporters';
 import {Credentials} from './credentials';
+import {Headers} from './oauth2client';
 
 export declare interface AuthClient {
   on(event: 'tokens', listener: (tokens: Credentials) => void): this;
 }
 
 export abstract class AuthClient extends EventEmitter {
+  protected quotaProjectId?: string;
   transporter = new DefaultTransporter();
   credentials: Credentials = {};
 
@@ -37,4 +39,25 @@ export abstract class AuthClient extends EventEmitter {
   setCredentials(credentials: Credentials) {
     this.credentials = credentials;
   }
+
+  /**
+   * Append additional headers, e.g., x-goog-user-project, shared across the
+   * classes inheriting AuthClient. This method should be used by any method
+   * that overrides getRequestMetadataAsync(), which is a shared helper for
+   * setting request information in both gRPC and HTTP API calls.
+   *
+   * @param headers objedcdt to append additional headers to.
+   */
+  protected addSharedMetadataHeaders(headers: Headers): Headers {
+    // quota_project_id, stored in application_default_credentials.json, is set in
+    // the x-goog-user-project header, to indicate an alternate account for
+    // billing and quota:
+    if (
+      !headers['x-goog-user-project'] && // don't override a value the user sets.
+      this.quotaProjectId
+    ) {
+      headers['x-goog-user-project'] = this.quotaProjectId;
+    }
+    return headers;
+  }
 }
diff --git a/src/auth/jwtclient.ts b/src/auth/jwtclient.ts
index a2153a27..e5ffa58d 100644
--- a/src/auth/jwtclient.ts
+++ b/src/auth/jwtclient.ts
@@ -128,7 +128,11 @@ export class JWT extends OAuth2Client {
         }).target_audience
       ) {
         const {tokens} = await this.refreshToken();
-        return {headers: {Authorization: `Bearer ${tokens.id_token}`}};
+        return {
+          headers: this.addSharedMetadataHeaders({
+            Authorization: `Bearer ${tokens.id_token}`,
+          }),
+        };
       } else {
         // no scopes have been set, but a uri has been provided. Use JWTAccess
         // credentials.
@@ -139,7 +143,7 @@ export class JWT extends OAuth2Client {
           url,
           this.additionalClaims
         );
-        return {headers};
+        return {headers: this.addSharedMetadataHeaders(headers)};
       }
     } else {
       return super.getRequestMetadataAsync(url);
diff --git a/src/auth/oauth2client.ts b/src/auth/oauth2client.ts
index e2bc61f4..5dae743f 100644
--- a/src/auth/oauth2client.ts
+++ b/src/auth/oauth2client.ts
@@ -383,7 +383,6 @@ export class OAuth2Client extends AuthClient {
   private certificateCache: Certificates = {};
   private certificateExpiry: Date | null = null;
   private certificateCacheFormat: CertificateFormat = CertificateFormat.PEM;
-  protected quotaProjectId?: string;
   protected refreshTokenPromises = new Map<string, Promise<GetTokenResponse>>();
 
   // TODO: refactor tests to make this private
@@ -808,18 +807,7 @@ export class OAuth2Client extends AuthClient {
     const headers: {[index: string]: string} = {
       Authorization: credentials.token_type + ' ' + tokens.access_token,
     };
-
-    // quota_project_id, stored in application_default_credentials.json, is set in
-    // the x-goog-user-project header, to indicate an alternate account for
-    // billing and quota:
-    if (
-      !headers['x-goog-user-project'] && // don't override a value the user sets.
-      this.quotaProjectId
-    ) {
-      headers['x-goog-user-project'] = this.quotaProjectId;
-    }
-
-    return {headers, res: r.res};
+    return {headers: this.addSharedMetadataHeaders(headers), res: r.res};
   }
 
   /**
diff --git a/test/fixtures/service-account-with-quota.json b/test/fixtures/service-account-with-quota.json
new file mode 100644
index 00000000..94ab5860
--- /dev/null
+++ b/test/fixtures/service-account-with-quota.json
@@ -0,0 +1,12 @@
+{
+    "type": "service_account",
+    "project_id": "my-account",
+    "private_key_id": "abc123",
+    "client_email": "fake@example.com",
+    "client_id": "222222222222222222222",
+    "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+    "token_uri": "https://oauth2.googleapis.com/token",
+    "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+    "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/fake@example.com",
+    "quota_project_id": "fake-quota-project"
+  }
diff --git a/test/test.googleauth.ts b/test/test.googleauth.ts
index 4ae37080..8987026f 100644
--- a/test/test.googleauth.ts
+++ b/test/test.googleauth.ts
@@ -1469,6 +1469,7 @@ describe('googleauth', () => {
     );
     const auth = new GoogleAuth();
     const client = await auth.getClient();
+    assert(client instanceof UserRefreshClient);
     const headers = await client.getRequestHeaders();
     assert.strictEqual(headers['x-goog-user-project'], 'my-quota-project');
     tokenReq.done();
@@ -1480,6 +1481,7 @@ describe('googleauth', () => {
     );
     const auth = new GoogleAuth();
     const client = await auth.getClient();
+    assert(client instanceof UserRefreshClient);
     const apiReq = nock(BASE_URL)
       .post(ENDPOINT)
       .reply(function(uri) {
diff --git a/test/test.jwt.ts b/test/test.jwt.ts
index 838f3de6..e3aecbc9 100644
--- a/test/test.jwt.ts
+++ b/test/test.jwt.ts
@@ -18,7 +18,7 @@ import * as jws from 'jws';
 import * as nock from 'nock';
 import * as sinon from 'sinon';
 
-import {JWT} from '../src';
+import {GoogleAuth, JWT} from '../src';
 import {CredentialRequest, JWTInput} from '../src/auth/credentials';
 
 const keypair = require('keypair');
@@ -810,3 +810,22 @@ it('getCredentials should handle a json keyFile', async () => {
   assert.strictEqual(private_key, json.private_key);
   assert.strictEqual(client_email, json.client_email);
 });
+
+it('getRequestHeaders populates x-goog-user-project for JWT client', async () => {
+  const auth = new GoogleAuth({
+    credentials: Object.assign(
+      require('../../test/fixtures/service-account-with-quota.json'),
+      {
+        private_key: keypair(1024 /* bitsize of private key */).private,
+      }
+    ),
+  });
+  const client = await auth.getClient();
+  assert(client instanceof JWT);
+  // If a URL isn't provided to authorize, the OAuth2Client super class is
+  // executed, which was already exercised.
+  const headers = await client.getRequestHeaders(
+    'http:/example.com/my_test_service'
+  );
+  assert.strictEqual(headers['x-goog-user-project'], 'fake-quota-project');
+});