In Petrolisthes violaceus, the last pereiopods are reduced, chelated, and highly movable. Several aggregations of specialised grooming setae are located mainly on the carpus, propodus, and dactylus of 5th pereiopods. Under laboratory conditions, ovigerous females were frequently observed to groom brooded embryos with this appendage. Grooming of the brood mass was most often observed during the day when brooding females remained relatively immobile, while embryo grooming was almost absent during the night when brooding females were otherwise highly active. When small shell pieces, dead embryos belonging to the same species, or small epoxy resin balls were inserted into the brood mass, the 5th pereiopods did not search for, manipulate, nor actively discard them. However, foreign objects were “incidentally removed” from the brood mass due to the preening of brooded embryos by the 5th pereiopods. Females that had the 5th pereiopods experimentally removed suffered higher embryo mortality during incubation than females with intact 5th pereiopods. Thus, brood grooming by 5th pereiopods significantly enhances survival of embryos during incubation. Compared to females with intact 5th pereiopods the hatching process of experimental females was significantly longer. Active brood care behaviour as described here for P. violaceus may also occur in other Anomuran species, because most of them bear a 5th pereiopod modified as a chelated grooming structure.