diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index a984dd9..ac59332 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -17,6 +17,22 @@ jobs:
       - uses: actions-rust-lang/setup-rust-toolchain@v1
       - run: cargo build --verbose
       - run: cargo test --verbose --all
+       
+build_old:
+    name: cargo build and test (packed_simd)
+    strategy:
+      matrix:
+        # Needs big runners to run tests
+        # Only macos-13-xlarge is Apple Silicon, as per:
+        # https://docs.github.com/en/actions/using-github-hosted-runners/about-larger-runners/about-larger-runners#about-macos-larger-runners
+        os: [ubuntu-22.04-github-hosted-16core, macos-13-xlarge]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      # Still compile the old rust nightly with packed simd - until we have a good replacement in poseidon.
+      - run: cargo +nightly-2023-05-31 build --features include_packed_simd
+      - run: cargo +nightly-2023-05-31 test --features include_packed_simd
 
   formatting:
     name: cargo fmt