The Pea Crab, Pinniza longpies, is commensal with the tube building polychaete, Axiothella rubrocincta. This worm constructs U-shaped tubes in san-mud substrata of bays and estuaries along the Pacific coast. The pronounced lateral elongation and diminutive size of p. Longpies enable it to move easily in the narrow tube with the worm. The species has occasionally been found with other polychaetes, including Pectinaria and Pista. According to Schmitt et al (1973) Pinnixa longpies range from Tomales Bay to Laguna Beach in California. There are no published accounts regarding the biology of this commensal crab. The present paper reports in detail on the larval development of P. longpies as reared under laboratory conditions. These results are compared with previous descriptions of Pinnotherid larvae.