We present ByteSTM, a virtual machine-level Java STM implementation that is built by extending the Jikes RVM. We modify Jikes RVM’s optimizing compiler to transparently support implicit transactions. Being implemented at the VM-level, it accesses memory directly, avoids Java garbage collection overhead by manually managing memory for transactional metadata, and provides pluggable support for implementing different STM algorithms to the VM. Our experimental studies reveal throughput improvement over other non-VM STMs by 6–70% on micro-benchmarks and by 7–60% on macro-benchmarks.