diff --git a/spack.yaml b/spack.yaml
index 0174c7862dae580370d760101f714c724a999d6d..e49a0a2866ee12f585aafc48837ac8282973098d 100644
--- a/spack.yaml
+++ b/spack.yaml
@@ -3,7 +3,7 @@ spack:
     # General
     - py-pip %gcc@10.3.0
     # EBRAINS simulators
-    - nest +python +gsl +mpi %gcc@10.3.0
+    - nest@2.20.0 +python +gsl +mpi %gcc@10.3.0
     - arbor +mpi %gcc@10.3.0
     - neuron +mpi %gcc@10.3.0
     # BrainScaleS (plus some constraining)