The Barbilla National Park, a natural area of high biodiversity on the Caribbean slope of Costa Rica, possesses two species of freshwater crabs that share the same biotope in different localities of the Park. The first species, Potamocarcinus magnus (Rathbun, 1896), one of the largest species of the family Pseudothelphusidae, is widely distributed in Middle America, from Costa Rica to Southern Mexico. The other species is a new species, Ptychophallus barbillaensis. This is a species of small crabs, possibly restricted to the National Park and neighboring areas. It can be distinguished from all other species in the genus by the form of the receptacle formed in the apex of the male gonopod, possibly for the retention of spermatophora during copulation. The species of Ptychophallus Smalley, can be arranged in a morphocline according to the relative development of this receptacle, with the present new species midway between the ancestral condition and the closed channel found in P. goldmanni Pretzmann, 1965.