Anphira guianensis sp. nov. (Isopoda, Cymothoidae) is described from the branchial cavity of a freshwater fish, Acnodon oligacanthus Eigenmann, 1903 from French Guiana. The new species differs from the other known species of the genus in a number of important respects. Adult females are less convex than those of the other two known species and the mouth points downward. The coxal plates are largest posteriorly, unlike those of the other species. The antennule is much stouter and longer than the antenna whereas in the other species these appendages are subequal. Additionally, the dactyls of the new species are much longer.
Cymothoidae; Isopod parasite; fish parasite; Brazil