1. Serological tests have revealed that the serum of the brachyuran crab Geryon quinquedens has a greater degree of correspondence to the sera of species in the families Xanthidae and Portunidae than to the sera of species in the families Cancridae, Ocypodidae, Grapsidae and Majidae.2. Of the species which were tested serologically, those in the families Xanthidae and Portunidae have the closest relationship. The family Cancridae is closer to the Xanthidae and Portunidae than any other of the families tested.3. The families Grapsidae, Ocypodidae, and Majidae are quite distinct from one another, and from the other families tested. The family Majidae is serologically least like any of the families tested.