All species in the epicaridean isopod genus ZonophryxusRichardson, 1903 are reviewed, and information regarding host choice is summarized. The enigmatic species Colypurus agassizi Richardson, 1905 is shown to be synonymous with Zonophryxus similisSearle, 1914, and Colypurus Richardson, 1905 and Colypuridae Richardson, 1905 are synonymized with Zonophryxus and Dajidae, respectively. The correct name for the species is Zonophryxus agassizi, new combination. The existence of two distinct male morphotypes in at least some species of Zonophryxus is confirmed and discussed. Type and other material of Zonophryxus retrodensRichardson, 1903 and Zonophryxus trilobusRichardson, 1910 were examined, and these species are discussed and illustrated, including description of morphological structures either erroneously or not previously reported. New material of Zonophryxus dodecapusHolthuis, 1949 is reported on, with the discovery that females of the species can have either five or six pairs of pereopods independent of their developmental stage. A new species, Zonophryxus probisowa, is described from Peruvian material and named in honor of The Proceedings of the Biological Society of Washington on the occasion of this, its final volume.