Crowbar is a new testing library for OCaml, combining QuickCheck-style property-based testing and directed fuzzing. Unlike other QuickCheck-style systems, Crowbar does not generate test cases purely at random: instead, the fuzzer afl-fuzz uses instrumentation automatically inserted into the program under test to provide feedback on the code paths triggered by a test case, in order to generate test cases exploring as much of the program’s behaviour as possible. Crowbar has found bugs in most of the software that has been tested with it.