Petrolisthes cabrilloi Glassell, 1945, is one of several species of porcelain crabs found along the coast of California. All species of Petrolisthes, like other members of the Porcellanidae, are filter-feeders, using the long hairs which fringe the third maxillipeds to capture food particles suspended in the water (Nicol, 1932). The species range of P. cabrilloi overlaps those of at least two other Petrolisthes species, P. gracilis Stimpson, 1859, to the south, P. eriomerus Stimpson, 1871, to the north. Other species of porcelain crabs, such as Pachycheles rudis Stimpson, 1859, are also found within the normal home range of Petrolisthes cabrilloi (cf. Haig, I960). It is not uncommon to find representatives of more than one species of Petrolisthes, as well as other porcellanid species of similar size, under the same rock. Under these conditions, it seems legitimate to ask whether these crabs are employing any type of behavioral resource partitioning in those areas where two or more species occur together. Though these crabs are common, behavioral observations in the field are sparse; understandably, as the crabs not only spend most of their time under rocks or under water, but also are nocturnally active. Molenock (unpublished) has made the only long-term attempt to observe behavior in P. cabrilloi, though observations of feeding behavior were limited to the laboratory. Only the usual filter-feeding behavior was noted in that study. In late March of 1975, I kept ten specimens of P. cabrilloi, together with two specimens of P. gracilis and four of Pachycheles rudis in a small indoor aquarium in La Jolla, California. The aquarium was furnished with fine beach sand and several small (4-6 cm in diameter) rounded rocks, collected from the same beach as the P. cabrilloi specimens. The individuals of all three species clustered beneath the rocks during most of the daylight hours, becoming slightly more active at twilight. High levels of activity did not occur until the lights were turned out at 10:00 PM. Regardless of the time of day, however, individual crabs would emerge onto the open spaces of sand and engage in filter-feeding (see Nicol, 1932, for description) whenever commercial fish-food in flake form was dropped into the water. At other times, a single crab would occasionally be found sitting quietly on the sand or filter feeding in a lackadaisical manner. Though similar in form, most species of Petrolisthes are readily distinguishable.