diff --git a/libcxx/test/support/filesystem_test_helper.h b/libcxx/test/support/filesystem_test_helper.h
--- a/libcxx/test/support/filesystem_test_helper.h
+++ b/libcxx/test/support/filesystem_test_helper.h
@@ -190,6 +190,12 @@
         auto large_file_fopen = fopen;
         auto large_file_ftruncate = utils::ftruncate;
         using large_file_offset_t = off_t;
+#elif defined(__BIONIC__)
+        // Bionic does not distinguish between fopen and fopen64, and it has an
+        // ftruncate64 that accepts off64_t.
+        auto large_file_fopen = fopen;
+        auto large_file_ftruncate = ftruncate64;
+        using large_file_offset_t = off64_t;
 #else
         auto large_file_fopen = fopen64;
         auto large_file_ftruncate = ftruncate64;