Feminization of adult male xanthid crabs, Tumidocarcinus giganteus Glaessner, from the Miocene of New Zealand, represents the first notice of the phenomenon in the fossil record. Marked broadening of the abdomen of adult males, to assume a form much like that of mature females and limitation in the size of the major claw likely result from parasitic castration by rhizocephalan, probably kentrogonid, barnacles. A sexually aberrant dakoticancrid, Dakoticancer overanus Rathbun, from the Late Cretaceous of South Dakota (Bishop, 1983) may extend this coevolutionary relationship between parasitic barnacles and decapod crustaceans into the Mesozoic.