diff --git a/pkgs/development/libraries/science/math/scalapack/default.nix b/pkgs/development/libraries/science/math/scalapack/default.nix
index 155e29df6c11..9a7374f4f49c 100644
--- a/pkgs/development/libraries/science/math/scalapack/default.nix
+++ b/pkgs/development/libraries/science/math/scalapack/default.nix
@@ -2,7 +2,7 @@
 , mpi, blas, lapack
 } :
 
-assert (!blas.isILP64) && (!lapack.isILP64);
+assert blas.isILP64 == lapack.isILP64;
 
 stdenv.mkDerivation rec {
   pname = "scalapack";
@@ -15,6 +15,18 @@ stdenv.mkDerivation rec {
     sha256 = "0hiap5i9ik6xpvl721n2slanlqygagc1pg2bcjb27ans6balhsfh";
   };
 
+  passthru = { inherit (blas) isILP64; };
+
+  # Required to activate ILP64.
+  # See https://github.com/Reference-ScaLAPACK/scalapack/pull/19
+  postPatch = lib.optionalString passthru.isILP64 ''
+    sed -i 's/INTSZ = 4/INTSZ = 8/g'   TESTING/EIG/* TESTING/LIN/*
+    sed -i 's/INTGSZ = 4/INTGSZ = 8/g' TESTING/EIG/* TESTING/LIN/*
+
+    # These tests are not adapted to ILP64
+    sed -i '/xssep/d;/xsgsep/d;/xssyevr/d' TESTING/CMakeLists.txt
+  '';
+
   nativeBuildInputs = [ cmake ];
   checkInputs = [ openssh ];
   buildInputs = [ blas lapack ];
@@ -28,6 +40,10 @@ stdenv.mkDerivation rec {
       -DLAPACK_LIBRARIES="-llapack"
       -DBLAS_LIBRARIES="-lblas"
       -DCMAKE_Fortran_COMPILER=${mpi}/bin/mpif90
+      ${lib.optionalString passthru.isILP64 ''
+        -DCMAKE_Fortran_FLAGS="-fdefault-integer-8"
+        -DCMAKE_C_FLAGS="-DInt=long"
+      ''}
       )
   '';
 
@@ -55,5 +71,4 @@ stdenv.mkDerivation rec {
     platforms = [ "x86_64-linux" ];
     maintainers = with maintainers; [ costrouc markuskowa ];
   };
-
 }