Small trematodes were observed by Dr G. S. Carter and Mr L. C. Beadle in the pericardiac cavities of Lepidosiren paradoxa caught in the swamp at Makthlawaiya in the Gran Chaco, during the months January to June 1927. The infected Lepidosiren varied in length from 20 to 34 ins., and constituted almost one-half of all the fish collected during that period. Dr Carter tells me that the infected fish appeared to be in perfect condition. Specimens from fifteen fish were preserved, and have been examined.