Riggia cryptocularis sp. nov. (Isopoda, Cymothoidae) is described on the basis of one female and one male specimen from the body cavity of freshwater fish from Mato Grosso do Sul State, Brazil. The new species is similar in size to R. paranensis Szidat, 1948, but differs from that species in the following respects. The body shapes are different since the widest part of the adult female is more anterior in R. paranensis. The eyes of the new species are covered and non-functional unlike those of the other species. The cephalon of the new species has a postero-dorsal elevation and a rounded and depressed frons which are lacking in the other species. Pereonites 5-7 are subequal in length in the new species and decrease in length in R. paranensis. The pleon/pleotelson tapers in the female of the new species and in the other it is bluntly rounded and with nearly parallel sides. The pleopods of R. cryptocularis sp. nov. also taper while those of R. paranensis are rounded. The recently described species, R. acuticaudata Thatcher, Lopes & Froehlich, 2002, was also obtained in Mato Grosso do Sul State but differs greatly from the presently described species. The body of R. acuticaudata is much smaller, and more rounded. The eyes of this species are large and functional, the dorsum of the cephalon lacks a dorsal prominence and the pleon/pleotelson tapers more abruptly

ABSTRACT. Riggia aculicaudala sp. novo is described on the basis of 10 female specimens recovered from the body cavity of the freshwater catfish, AncislntS sp . The fish were caught in a stream called the Córrego Piraputanga, a tributary of the Paraguai River. The new species differs from the others in the genus by being smaller and less convex and by havi ng a pleotelson that tapers abruptly ratherthan being the same width throughout. AIso, the pereopods and dactyls are much smaller, the maxillule has two terminal and three subterminal spines and the first two pleopods are elongate and acute. Additionally. R. aculÍcaudata sp. novo is from a different Order of fish host and a new geographic area. KEY WORDS. Isopod parasite, cymothoid, fish parasite. Brazil