diff --git a/t/test.pl b/t/test.pl
index fc5b215a..7f17f107 100644
--- a/t/test.pl
+++ b/t/test.pl
@@ -11,6 +11,15 @@ BEGIN {
     $App::Yath::Script::SCRIPT = which 'yath';
 }
 use App::Yath::Util qw/find_yath/;
+use List::SomeUtils qw(none);
+
+if (defined($ENV{"NIX_BUILD_CORES"})
+    and not defined($ENV{"YATH_JOB_COUNT"})
+    and not defined($ENV{"T2_HARNESS_JOB_COUNT"})
+    and not defined($ENV{"T2_HARNESS_JOB_COUNT"})) {
+    $ENV{"YATH_JOB_COUNT"} = $ENV{"NIX_BUILD_CORES"};
+    print STDERR "test.pl: Defaulting \$YATH_JOB_COUNT to \$NIX_BUILD_CORES (${\$ENV{'NIX_BUILD_CORES'}})\n";
+}
 
 system($^X, find_yath(), '-D', 'test', '--default-search' => './', @ARGV);
 my $exit = $?;