Abstract: This paper describes the design, implementation and performance of the NX message-passing interface on the Shrimp multicomputer. Unlike traditional methods, our implementation, exploiting Shrimp's virtual memory-mapped communication facility, performs buffer management at user level without using a special message-passing processor, and requires no CPU intervention upon message arrival in the common cases. For a four-byte message, our implementation, achieves a user-to-user latency of 12 microseconds, about factor of four smaller than that on the Intel Paragon. For large messages, our implementation quickly approaches the bandwidth limit imposed by the Shrimp hardware.