diff --git a/doc/writing_a_test.md b/doc/writing_a_test.md
index 5b58775ecee96d54a0b85ff558600fffa87f27cd..4d7425d64bb5e5a07f32602adf58bcd98759c9cc 100644
--- a/doc/writing_a_test.md
+++ b/doc/writing_a_test.md
@@ -227,6 +227,12 @@ A `pyfeeds` test is called with three parameters:
      previously-generated benchmark data for the test, if it exists.
 
 
+When `pyfeeds` calls a `feedsRun` script, it sets an environment variable -
+`PYFEEDS_TESTING=1`. This may be useful for `feedsRun` scripts which are intended
+to be callable both from `pyfeeds`, and through other means, and which need to
+behave differently depending on how they are called.
+
+
 ## Writing your test in Python