diff --git a/file.go b/file.go
index d1143f6e1..528eaafb6 100644
--- a/file.go
+++ b/file.go
@@ -1271,6 +1271,7 @@ func (f *File) addFileHeaderData(file *File) *File {
 	file.Header.ImmediateDestination = f.Header.ImmediateDestination
 	file.Header.FileCreationDate = time.Now().Format("060102")
 	file.Header.FileCreationTime = time.Now().AddDate(0, 0, 1).Format("1504") // HHmm
+	file.Header.FileIDModifier = f.Header.FileIDModifier
 	file.Header.ImmediateDestinationName = f.Header.ImmediateDestinationName
 	file.Header.ImmediateOriginName = f.Header.ImmediateOriginName
 	return file
diff --git a/file_test.go b/file_test.go
index 035a447b3..9518c3b65 100644
--- a/file_test.go
+++ b/file_test.go
@@ -2219,3 +2219,25 @@ func TestFileFromJSON_ValidateOpts(t *testing.T) {
 	require.NotNil(t, validateOpts)
 	require.True(t, validateOpts.PreserveSpaces)
 }
+
+func TestFile_FlattenBatches_PreservesFileIDModifier(t *testing.T) {
+	f := NewFile()
+	f.SetHeader(mockFileHeader())
+	f.Header.FileIDModifier = "B"
+	batch1 := mockBatchPPD(t)
+	f.AddBatch(batch1)
+
+	err := f.Create()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	flattened, err := f.FlattenBatches()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if flattened.Header.FileIDModifier != "B" {
+		t.Errorf("FileIDModifier not preserved: want 'B' got %s", flattened.Header.FileIDModifier)
+	}
+}