Examination of 186 specimens of the living brachiopod Laqueus rubellus (Sowerby), collected during four different months from Sagami Bay, Japan, has yielded 94 specimens of the pinnotherid crab, Pinnotheres laquei Sakai, living within the mantle cavity. This work provides the first quantitative statements regarding crab/brachiopod associations. All but four of the specimens were female crabs; the four male specimens occurred together with females in brachiopods collected during March, 1982. Pinnotherids, common associates of mollusks and echinoderms, tend to be host specific; however, anecdotal information received near the conclusion of this study suggests that P. laquei may visit multiple brachiopod host species. Although the crabs seem to be more common in larger brachiopods, there is some correspondence between crab size and brachiopod size. No physical evidence, such as malformation of the shell or lophophore loop, was observed in host brachiopods and, therefore, the association was judged to be commensal or symbiotic. It is probable that crabs were able to enter and exit the mantle cavity at will. This association of brachiopod and pinnotherid represents the only known occurrence of infestation of brachiopods by crabs and, with the exception of boring and encrusting organisms, is one of very few biotic associations, of any kind, for brachiopods. Although this association has not yet been reported from the fossil record, the nature of preservation of brachiopods and the methods of study, employing serial sectioning techniques, suggest the possibility of detecting this unusual association in the fossil record.