ABSTRACT: In southern Puget Sound, large numbers of megalopae and juveniles of the brachyuran crab Cancer gracilis and the hyperiid amphipod Hyperia medusarum were found riding the scyphozoan Phacellophora camtschatica. C. gracilis megalopae numbered up to 326 individuals per medusa, instars reached 13 individuals per host and H. medusarum numbered up to 446 amphipods per host. Although C. gracilis megalopae and instars are not seen riding Aurelia labiata in the field, instars readily clung to A. labiata, as well as an artificial medusa, when confined in a planktonkreisel. In the laboratory, C. gracilis was observed to consume H. medusarum, P. camtschatica, Artemia franciscana and A. labiata. Crab fecal pellets contained mixed crustacean exoskeletons (70%), nematocysts (20%), and diatom frustules (8%). Nematocysts predominated in the fecal pellets of all stages and sexes of H. medusarum. In stable isotope studies, the δ¹³C and δ¹⁵N values for the megalopae (–19.9 and 11.4, respectively) fell closely in the range of those for H. medusarum (–19.6 and 12.5, respectively) and indicate a similar trophic reliance on the host. The broad range of δ¹³C (–25.2 to –19.6) and δ¹⁵N (10.9 to 17.5) values among crab instars reflects an increased diversity of diet as crabs develop. The association between C. gracilis and P. camtschatica is unusual because of the ontogenic switch of the symbiont from a primarily kleptoparasitic association to a facultative cleaning association. It is suggested that P. camtschatica incidentially concentrates H. medusarum in its oral arms as the symbionts transfer from its gelatinous prey. Metabolic studies suggest that by riding medusae, crabs may be able to develop faster through transport into warmer surface waters while reducing the energetic costs associated with locomotion.