The porcelain crabs Petrol~sthes cinctipes (Randall) and P. eriomerus Stimpson are sympatric on exposed rocky shores in the northeastern Pacific These species often exhibit a striking pattern of non-overlapping vert~cal distributions, wlth P. eriomerus inhabiting low Intertidal and subtidal areas and P. cinctipes occurring in the middle and high intert~dal. Transplant experiments revealed that P. enomerus was unable to tolerate displacement as l~ttle as 0 3 m above ~t s normal limit, while in contrast its congener showed good survival and growth when caged at lower levels. The 2 species were equally susceptible to desiccat~on; however, P. enomerus was extremely sens~tive to thermal stress during periods of aerial exposure, and thermal tolerance varied inversely with size. Abiot~c factors also control the d~stnbution of P. cinctipes; t h~s species was competitively dominant in laboratory experiments, suggesting the lower limit is not due to ~nteractions w~t h its congener. Rather, this d~s t n b u t~o n is l~n k e d to sudden changes in substrate composition, involving an actlve avo~dance of rocks resting on sand or s~l t as demonstrated through f~e l d manipulation of substrate beneath rock patches. Patterns of resource (space) p a r t~t i o n~n g beneath rocks are largely a passive consequence of phys~olog~cal constraints and substrate preferences by adults, maintamed In part through gregarious settlement by megalopae of both species