Anchistus custos Forskål is the only common prawn commensal with bivalves of the family Pinnidae in Singapore waters. The whole morphology shows pronounced modifications connected with the commensal mode of life. The second legs are large and massive and show marked positive allometry in the male but appear to have little function in the general life of the animal. The animal occurs wherever there are established populations of matureindividuals of its host. Infestation rates are heavy and are affected by size of host and probably by competition with pinnotherid crabs. Multiple infestations are common and always involve individuals of both sexes and it has not been possible to demonstrate aggressive or territorial behaviour. Reproduction appears to occur throughout the year. Individuals are not normally found away from their hosts but rapidly abandon dead hosts. Location of the host depends in part on tactile stimuli and thigmotaxy and in part on pronounced positive rheotaxy. It has not been possible to demonstrate any distance chemical sense involved in host location but the prawn is repulsed by the presence of dead Pinna flesh. The prawn normally clings to the edge of the gill lamellae of its host and feeds by means of the highly modified first legs which are used as scrapers to remove mucus and entangled food particles from the gills of the host. Examination of gut contents of the prawn confirms this deduction. There is no evidence for any harmful effects on the host.