PGAS languages aim to enhance productivity for large scale systems. The IBM Asynchronous PGAS runtime (APGAS) supports various high productivity programming languages including UPC, X10 and CAF. The runtime has been designed for scalability and performance portability, and it includes optimized implementations for LAPI and Blue Gene DCMF communication sub systems. This paper presents an optimized implementation of the IBM APGAS runtime for Myrinet networks, on top of the MX communication library. It explains the challenges of implementing a one-sided communication model (APGAS) on top of a two-sided communication API such as MX. We show that our implementation outperforms the Berkeley GASNet runtime in terms of latency and bandwidth. We also demonstrate scalability of various HPC benchmarks up to 1024 processes.