diff --git a/pkgs/by-name/br/brlcad/package.nix b/pkgs/by-name/br/brlcad/package.nix
index 778c2a736045..959372e358fd 100644
--- a/pkgs/by-name/br/brlcad/package.nix
+++ b/pkgs/by-name/br/brlcad/package.nix
@@ -187,12 +187,21 @@ stdenv.mkDerivation (finalAttrs: {
         build/bext/tktable/tktable/CMake/FindTCL.cmake \
         build/bext/tkhtml/tkhtml/CMake/FindTCL.cmake
     ''
-    # remove a failing test
+    # remove failing tests
     + lib.optionalString stdenv.hostPlatform.isAarch64 ''
       substituteInPlace src/libbu/tests/CMakeLists.txt \
         --replace-fail \
           "brlcad_add_test(NAME bu_color_to_rgb_floats_1 COMMAND bu_test test_color 4 192,78,214)" \
           ""
+    ''
+    # This test opens 16k mapped files from every CPU, each open being a linear scan
+    # under one global semaphore, so its runtime explodes on many-core machines and
+    # blows past ctest's 1500s default timeout on our 80-core aarch64 builders.
+    + lib.optionalString stdenv.hostPlatform.isAarch64 ''
+      substituteInPlace src/libbu/tests/CMakeLists.txt \
+        --replace-fail \
+          "brlcad_add_test(NAME bu_mappedfile_parallel_16384 COMMAND bu_test test_mappedfile 2 16384)" \
+          ""
     '';
 
   nativeBuildInputs = [