The authors study the Brazilian material of the genus Prosthogonimus Luehe, 1899, deposited in the Helminthological Collection of the Oswaldo Cruz Institute, determined and without determination and conclude: although showing a great variation in its characters, it belongs to a single species: Prosthogonimus ovalus (Rudolphi, 1803). This variation was remarked by Travassos (1922 and 1928) and experimentally confirmed by Boddeke (1960). Forty nine figures and five measurement tables are presented, showing all these variations.