diff --git a/cranelift/docs/testing.md b/cranelift/docs/testing.md
index bcbe3bdea8..92dc99f33a 100644
--- a/cranelift/docs/testing.md
+++ b/cranelift/docs/testing.md
@@ -74,6 +74,15 @@ run will also have the RISC-V specific flag `supports_m` disabled.
 The filetests are run automatically as part of `cargo test`, and they can
 also be run manually with the `clif-util test` command.
 
+By default, the test runner will spawn a thread pool with as many threads as
+there are logical CPUs. You can explicitly control how many threads are spawned
+via the `CRANELIFT_FILETESTS_THREADS` environment variable. For example, to
+limit the test runner to a single thread, use:
+
+```
+$ CRANELIFT_FILETESTS_THREADS=1 clif-util test path/to/file.clif
+```
+
 ### Filecheck
 
 Many of the test commands described below use *filecheck* to verify their
diff --git a/cranelift/filetests/src/concurrent.rs b/cranelift/filetests/src/concurrent.rs
index 30e2c94cfe..67592ed03e 100644
--- a/cranelift/filetests/src/concurrent.rs
+++ b/cranelift/filetests/src/concurrent.rs
@@ -50,7 +50,16 @@ impl ConcurrentRunner {
 
         heartbeat_thread(reply_tx.clone());
 
-        let handles = (0..num_cpus::get())
+        let num_threads = std::env::var("CRANELIFT_FILETESTS_THREADS")
+            .ok()
+            .map(|s| {
+                use std::str::FromStr;
+                let n = usize::from_str(&s).unwrap();
+                assert!(n > 0);
+                n
+            })
+            .unwrap_or_else(|| num_cpus::get());
+        let handles = (0..num_threads)
             .map(|num| worker_thread(num, request_mutex.clone(), reply_tx.clone()))
             .collect();