Many porcelain crabs have an unusual claw (cheliped) form that is broad, flat and thin. This seemingly flimsy form suggests claws may be used more for display than for feeding or aggressive interactions. However, in the flat-topped porcelain crab Petrolisthes cinctipes, one of the most abundant intertidal crustaceans on the Pacific coast of North America, we observed a high proportion of individuals with puncture wounds on their claws. Claws therefore do appear to be used in intra-specific aggression. To examine factors that might influence injury prevalence, samples of P. cinctipes were collected from four sites in Barkley Sound, two wave-exposed, and two protected. In addition, laboratory experiments examined the effects of density and crab size on the incidence of puncture wounds. Crabs from wave-exposed, high-density sites and smaller crabs both exhibited significantly higher proportions of puncture wounds. Although not statistically significant, laboratory experiments revealed a trend towards a higher incidence of puncture wounds in high-density groups. Wound frequency did not differ between the sexes, so these injuries are not likely a result of intra- or inter-sexual interactions. Puncture wounds were concentrated on claw fingers (either the dactyl or propus), and along the ventral margin of the manus, both of which are likely to be nearest an opponent during shoving interactions. Overall, these data suggest that the claws of P. cinctipes mediate intra-specific competitive interactions that frequently escalate to injury.