Recognition of two different configurations of the abdomen in the Eocene raninid crab, Lophoranina bishopi Squires and Demetrion, 1992, from the Tepetate Formation in Baja California Sur, Mexico, represents the first published notice of sexual dimorphism in fossil representatives of the Ranininae. Examination of extant representatives of the six extant subfamilies of Raninidae documents sexually dimorphic characters of the abdomen throughout the family, although the style and degree of development of dimorphic features varies throughout the subfamilies. Recognition of patterns of sexual dimorphism of abdomina in the Raninidae is particularly useful in the fossil record where conventional means of determining gender are not preserved.