Acrothoracican barnacles of the genus Trypetesa are obligate symbionts of hermit crabs that burrow into the gastropod shells occupied by their hosts. In the present study, hermit crabs were examined for the presence of trypetesids, based on collections from the United States, Jamaica, and the Philippines made between 1997 and 2008. Shells from Jamaica and New York contained Trypetesa lateralis, a trypetesid previously documented from central California. Trypetesa lateralis is redescribed based on light and scanning electron microscopy, showing the presence of an external mantle flap and asymmetrical opercular bars diagnostic for this species. The mean prevalence of trypetesids in Jamaica was 8.3% and most barnacles were associated with Calcinus tibicen; in New York the barnacles were found in 1.6% of shells occupied by Pagurus longicarpus. Specimens from the Philippines were identified as Trypetesa spinulosa (formerly known only from Madagascar) based on the presence of their diagnostic orificial palps. The mean prevalence of T. spinulosa in the Philippines was 3.7% and most barnacles were associated with Calcinus spp. Hermit crab eggs were observed in the guts of T. lateralis from Jamaica and T. spinulosa from the Philippines. In both of these regions the trypetesids were found significantly more often in shells occupied by female hermit crab hosts (80–87% with females). These findings suggest the barnacles be classified as transient parasites. The biology of trypetesids is reviewed and a key to the family is provided. Further studies are needed to determine if egg predation occurs in all trypetesids and the impacts on hosts.