diff --git a/idtoken/idtoken.go b/idtoken/idtoken.go
index be52f3b2905..be2ba5cd429 100644
--- a/idtoken/idtoken.go
+++ b/idtoken/idtoken.go
@@ -104,7 +104,7 @@ func tokenSourceFromBytes(ctx context.Context, data []byte, audience string, ds
 	if err := isServiceAccount(data); err != nil {
 		return nil, err
 	}
-	cfg, err := google.JWTConfigFromJSON(data, ds.Scopes...)
+	cfg, err := google.JWTConfigFromJSON(data, ds.GetScopes()...)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/creds.go b/internal/creds.go
index dc6d50e96aa..c93daa98c32 100644
--- a/internal/creds.go
+++ b/internal/creds.go
@@ -34,24 +34,24 @@ func baseCreds(ctx context.Context, ds *DialSettings) (*google.Credentials, erro
 		return ds.Credentials, nil
 	}
 	if ds.CredentialsJSON != nil {
-		return credentialsFromJSON(ctx, ds.CredentialsJSON, ds.Endpoint, ds.Scopes, ds.Audiences)
+		return credentialsFromJSON(ctx, ds.CredentialsJSON, ds)
 	}
 	if ds.CredentialsFile != "" {
 		data, err := ioutil.ReadFile(ds.CredentialsFile)
 		if err != nil {
 			return nil, fmt.Errorf("cannot read credentials file: %v", err)
 		}
-		return credentialsFromJSON(ctx, data, ds.Endpoint, ds.Scopes, ds.Audiences)
+		return credentialsFromJSON(ctx, data, ds)
 	}
 	if ds.TokenSource != nil {
 		return &google.Credentials{TokenSource: ds.TokenSource}, nil
 	}
-	cred, err := google.FindDefaultCredentials(ctx, ds.Scopes...)
+	cred, err := google.FindDefaultCredentials(ctx, ds.GetScopes()...)
 	if err != nil {
 		return nil, err
 	}
 	if len(cred.JSON) > 0 {
-		return credentialsFromJSON(ctx, cred.JSON, ds.Endpoint, ds.Scopes, ds.Audiences)
+		return credentialsFromJSON(ctx, cred.JSON, ds)
 	}
 	// For GAE and GCE, the JSON is empty so return the default credentials directly.
 	return cred, nil
@@ -66,12 +66,12 @@ const (
 //
 // - If the JSON is a service account and no scopes provided, returns self-signed JWT auth flow
 // - Otherwise, returns OAuth 2.0 flow.
-func credentialsFromJSON(ctx context.Context, data []byte, endpoint string, scopes []string, audiences []string) (*google.Credentials, error) {
-	cred, err := google.CredentialsFromJSON(ctx, data, scopes...)
+func credentialsFromJSON(ctx context.Context, data []byte, ds *DialSettings) (*google.Credentials, error) {
+	cred, err := google.CredentialsFromJSON(ctx, data, ds.GetScopes()...)
 	if err != nil {
 		return nil, err
 	}
-	if len(data) > 0 && len(scopes) == 0 {
+	if len(data) > 0 && len(ds.Scopes) == 0 && (ds.DefaultAudience != "" || len(ds.Audiences) > 0) {
 		var f struct {
 			Type string `json:"type"`
 			// The rest JSON fields are omitted because they are not used.
@@ -80,7 +80,7 @@ func credentialsFromJSON(ctx context.Context, data []byte, endpoint string, scop
 			return nil, err
 		}
 		if f.Type == serviceAccountKey {
-			ts, err := selfSignedJWTTokenSource(data, endpoint, audiences)
+			ts, err := selfSignedJWTTokenSource(data, ds.DefaultAudience, ds.Audiences)
 			if err != nil {
 				return nil, err
 			}
@@ -90,9 +90,8 @@ func credentialsFromJSON(ctx context.Context, data []byte, endpoint string, scop
 	return cred, err
 }
 
-func selfSignedJWTTokenSource(data []byte, endpoint string, audiences []string) (oauth2.TokenSource, error) {
-	// Use the API endpoint as the default audience
-	audience := endpoint
+func selfSignedJWTTokenSource(data []byte, defaultAudience string, audiences []string) (oauth2.TokenSource, error) {
+	audience := defaultAudience
 	if len(audiences) > 0 {
 		// TODO(shinfan): Update golang oauth to support multiple audiences.
 		if len(audiences) > 1 {
@@ -118,7 +117,7 @@ func QuotaProjectFromCreds(cred *google.Credentials) string {
 
 func impersonateCredentials(ctx context.Context, creds *google.Credentials, ds *DialSettings) (*google.Credentials, error) {
 	if len(ds.ImpersonationConfig.Scopes) == 0 {
-		ds.ImpersonationConfig.Scopes = ds.Scopes
+		ds.ImpersonationConfig.Scopes = ds.GetScopes()
 	}
 	ts, err := impersonate.TokenSource(ctx, creds.TokenSource, ds.ImpersonationConfig)
 	if err != nil {
diff --git a/internal/creds_test.go b/internal/creds_test.go
index 0be8e2ae59a..045c5bc20a6 100644
--- a/internal/creds_test.go
+++ b/internal/creds_test.go
@@ -119,7 +119,7 @@ const validServiceAccountJSON = `{
 func TestQuotaProjectFromCreds(t *testing.T) {
 	ctx := context.Background()
 
-	cred, err := credentialsFromJSON(ctx, []byte(validServiceAccountJSON), "foo.googleapis.com", nil, nil)
+	cred, err := credentialsFromJSON(ctx, []byte(validServiceAccountJSON), &DialSettings{Endpoint: "foo.googleapis.com"})
 	if err != nil {
 		t.Fatalf("got %v, wanted no error", err)
 	}
@@ -133,7 +133,7 @@ func TestQuotaProjectFromCreds(t *testing.T) {
 	"quota_project_id": "foobar"
 }`)
 
-	cred, err = credentialsFromJSON(ctx, []byte(quotaProjectJSON), "foo.googleapis.com", nil, nil)
+	cred, err = credentialsFromJSON(ctx, []byte(quotaProjectJSON), &DialSettings{Endpoint: "foo.googleapis.com"})
 	if err != nil {
 		t.Fatalf("got %v, wanted no error", err)
 	}
diff --git a/internal/settings.go b/internal/settings.go
index 7c0f9292d50..0ae1cb9778d 100644
--- a/internal/settings.go
+++ b/internal/settings.go
@@ -23,6 +23,7 @@ type DialSettings struct {
 	DefaultEndpoint     string
 	DefaultMTLSEndpoint string
 	Scopes              []string
+	DefaultScopes       []string
 	TokenSource         oauth2.TokenSource
 	Credentials         *google.Credentials
 	CredentialsFile     string // if set, Token Source is ignored.
@@ -30,6 +31,7 @@ type DialSettings struct {
 	UserAgent           string
 	APIKey              string
 	Audiences           []string
+	DefaultAudience     string
 	HTTPClient          *http.Client
 	GRPCDialOpts        []grpc.DialOption
 	GRPCConn            *grpc.ClientConn
@@ -49,6 +51,15 @@ type DialSettings struct {
 	RequestReason string
 }
 
+// GetScopes returns the user-provided scopes, if set, or else falls back to the
+// default scopes.
+func (ds *DialSettings) GetScopes() []string {
+	if len(ds.Scopes) > 0 {
+		return ds.Scopes
+	}
+	return ds.DefaultScopes
+}
+
 // Validate reports an error if ds is invalid.
 func (ds *DialSettings) Validate() error {
 	if ds.SkipValidation {
diff --git a/option/internaloption/internaloption.go b/option/internaloption/internaloption.go
index 6ee4501c048..1fff22fd5da 100644
--- a/option/internaloption/internaloption.go
+++ b/option/internaloption/internaloption.go
@@ -65,3 +65,32 @@ type enableDirectPath bool
 func (e enableDirectPath) Apply(o *internal.DialSettings) {
 	o.EnableDirectPath = bool(e)
 }
+
+// WithDefaultAudience returns a ClientOption that specifies a default audience
+// to be used as the audience field ("aud") for the JWT token authentication.
+//
+// It should only be used internally by generated clients.
+func WithDefaultAudience(audience string) option.ClientOption {
+	return withDefaultAudience(audience)
+}
+
+type withDefaultAudience string
+
+func (w withDefaultAudience) Apply(o *internal.DialSettings) {
+	o.DefaultAudience = string(w)
+}
+
+// WithDefaultScopes returns a ClientOption that overrides the default OAuth2
+// scopes to be used for a service.
+//
+// It should only be used internally by generated clients.
+func WithDefaultScopes(scope ...string) option.ClientOption {
+	return withDefaultScopes(scope)
+}
+
+type withDefaultScopes []string
+
+func (w withDefaultScopes) Apply(o *internal.DialSettings) {
+	o.DefaultScopes = make([]string, len(w))
+	copy(o.DefaultScopes, w)
+}