The porcellanid crabs are diverse and abundant species inhabiting the intertidal and subtidal rocky substrate. One particular species is Allopetrolisthes punctatus, a medium-sized crab, poorly studied but strikingly abundant in some places of Chile. Near Chome (central Chile) large aggregations have been reported by local fishermen. A direct population assessment with semi-autonomous scuba diving was performed to evaluate stock structure, distribution, and abundance. A monthly monitoring was established to analyze the temporal variability of biological traits during a one-year time period. Dense crab aggregations were observed over the naked unprotected rocky substrate, with corporal size and sexual proportion spatially segregated. Allopetrolisthes punctatus showed a relatively high fecundity (164-8,741 eggs) and uninterrupted yearly spawning, which seems to favor the quick and successful settlement for this species in Chome.