A new nematode, Myleusnema bicumis gen. et sp. п., is described from the intestine of a characoid freshwater fish, Myleus ternetzi (Norman, 1929) (Cypriniformes: Serrasalmidae), from French Guiana. This cosmocercoid nematode species represents a new genus of the family Kathlaniidae, being characterized mainly by the shape of the body (the cephalic portion is separated from the rest of the body at the level of the nerve ring), structure of the cephalic extremity (presence of three lips and three lamella-like formations demarcating the buccal cavity), absence of an oesophageal pharynx, presence of medium-sized alate spicules (0.694-0.721 mm long), two conspicuous postcloacal homs associated with a gubernaculum, and by the number and distribution of caudal papillae in the male.