diff --git a/src/auth/authclient.ts b/src/auth/authclient.ts
index 3e443334..f41a1537 100644
--- a/src/auth/authclient.ts
+++ b/src/auth/authclient.ts
@@ -15,7 +15,7 @@
 import {EventEmitter} from 'events';
 import {GaxiosOptions, GaxiosPromise, GaxiosResponse} from 'gaxios';
 
-import {DefaultTransporter} from '../transporters';
+import {DefaultTransporter, Transporter} from '../transporters';
 import {Credentials} from './credentials';
 import {Headers} from './oauth2client';
 
@@ -93,7 +93,7 @@ export abstract class AuthClient
    * See {@link https://cloud.google.com/docs/quota| Working with quotas}
    */
   quotaProjectId?: string;
-  transporter = new DefaultTransporter();
+  transporter: Transporter = new DefaultTransporter();
   credentials: Credentials = {};
   projectId?: string | null;
   eagerRefreshThresholdMillis = 5 * 60 * 1000;
diff --git a/src/auth/externalAccountAuthorizedUserClient.ts b/src/auth/externalAccountAuthorizedUserClient.ts
index c0c081cd..ca267bda 100644
--- a/src/auth/externalAccountAuthorizedUserClient.ts
+++ b/src/auth/externalAccountAuthorizedUserClient.ts
@@ -20,7 +20,7 @@ import {
   OAuthClientAuthHandler,
   OAuthErrorResponse,
 } from './oauth2common';
-import {BodyResponseCallback, DefaultTransporter} from '../transporters';
+import {BodyResponseCallback, Transporter} from '../transporters';
 import {
   GaxiosError,
   GaxiosOptions,
@@ -83,7 +83,7 @@ class ExternalAccountAuthorizedUserHandler extends OAuthClientAuthHandler {
    */
   constructor(
     private readonly url: string,
-    private readonly transporter: DefaultTransporter,
+    private readonly transporter: Transporter,
     clientAuthentication?: ClientAuthentication
   ) {
     super(clientAuthentication);
diff --git a/src/auth/stscredentials.ts b/src/auth/stscredentials.ts
index 52178b82..19ec95bd 100644
--- a/src/auth/stscredentials.ts
+++ b/src/auth/stscredentials.ts
@@ -15,7 +15,7 @@
 import {GaxiosError, GaxiosOptions, GaxiosResponse} from 'gaxios';
 import * as querystring from 'querystring';
 
-import {DefaultTransporter} from '../transporters';
+import {DefaultTransporter, Transporter} from '../transporters';
 import {Headers} from './oauth2client';
 import {
   ClientAuthentication,
@@ -131,7 +131,7 @@ export interface StsSuccessfulResponse {
  * https://tools.ietf.org/html/rfc8693
  */
 export class StsCredentials extends OAuthClientAuthHandler {
-  private transporter: DefaultTransporter;
+  private transporter: Transporter;
 
   /**
    * Initializes an STS credentials instance.