Producing robust memory manager implementa- tions is a challenging task. Defects in garbage collection algo- rithms produce subtle effects that are revealed later in program execution as memory corruptions. This problem is exacerbated by the fact that garbage collection algorithms deal with low-level implementation details to be efficient. Finding, reproducing, and debugging such bugs is complex and time-consuming. In this article, we propose to fuzz heaps by generating large sequences of random heap events directed by virtual machine ex- perts. Randomly generated events exercise the garbage collection algorithm with the objective of crashing the virtual machine and finding bugs. Once a bug is found, we use a test case reduction algorithm that finds the smaller subset of events that reproduces the issue. We implemented our approach on top of the virtual machine simulator of the Pharo Virtual Machine, to test its sequential stop-the-world generational scavenger. We directed our fuzzing toward the ephemeron finalization mechanism, corner allocation cases, and the heap compaction algorithm. Our prototype found 6 bugs: 3 in Pharo’s ephemeron implementation which is not yet in production, 2 bugs in the default compactor which has been in production for 8 years, and 1 bug in the VM simulator used daily by VM developers. We show how such test cases were automatically reduced to trivial sequences that were easy to debug.