diff --git a/main.go b/main.go
index 295aeb1..9cb2e27 100644
--- a/main.go
+++ b/main.go
@@ -205,7 +205,7 @@ func licenseHeader(path string, tmpl *template.Template, data *copyrightData) ([
 	switch fileExtension(path) {
 	default:
 		return nil, nil
-	case ".c", ".h":
+	case ".c", ".h", ".gv":
 		lic, err = prefix(tmpl, data, "/*", " * ", " */")
 	case ".js", ".mjs", ".cjs", ".jsx", ".tsx", ".css", ".tf", ".ts":
 		lic, err = prefix(tmpl, data, "/**", " * ", " */")
diff --git a/testdata/expected/file.gv b/testdata/expected/file.gv
new file mode 100644
index 0000000..2c754ab
--- /dev/null
+++ b/testdata/expected/file.gv
@@ -0,0 +1,25 @@
+/*
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+graph ethane {
+    C_0 -- H_0;
+    C_0 -- H_1;
+    C_0 -- H_2;
+    C_0 -- C_1;
+    C_1 -- H_3;
+    C_1 -- H_4;
+    C_1 -- H_5;
+} 
diff --git a/testdata/initial/file.gv b/testdata/initial/file.gv
new file mode 100644
index 0000000..edaf8fd
--- /dev/null
+++ b/testdata/initial/file.gv
@@ -0,0 +1,9 @@
+graph ethane {
+    C_0 -- H_0;
+    C_0 -- H_1;
+    C_0 -- H_2;
+    C_0 -- C_1;
+    C_1 -- H_3;
+    C_1 -- H_4;
+    C_1 -- H_5;
+}