diff --git a/cranelift/test-all.sh b/cranelift/test-all.sh
index 6a9291d82e..2402eb7a39 100755
--- a/cranelift/test-all.sh
+++ b/cranelift/test-all.sh
@@ -11,6 +11,10 @@ set -euo pipefail
 #
 # All tests run by this script should be passing at all times.
 
+# Disable generation of .pyc files because they cause trouble for vendoring
+# scripts, and this is a build step that isn't run very often anyway.
+export PYTHONDONTWRITEBYTECODE=1
+
 # Repository top-level directory.
 cd $(dirname "$0")
 topdir=$(pwd)
diff --git a/lib/cretonne/build.rs b/lib/cretonne/build.rs
index d2ef3905b1..69be14de94 100644
--- a/lib/cretonne/build.rs
+++ b/lib/cretonne/build.rs
@@ -59,8 +59,11 @@ fn main() {
     let build_script = meta_dir.join("build.py");
 
     // Launch build script with Python. We'll just find python in the path.
+    // Use -B to disable .pyc files, because they cause trouble for vendoring
+    // scripts, and this is a build step that isn't run very often anyway.
     let status = process::Command::new("python")
         .current_dir(crate_dir)
+        .arg("-B")
         .arg(build_script)
         .arg("--out-dir")
         .arg(out_dir)