Genetic differentiation was studied among Leporinus friderici, L. granti, L. lebaili and L. affinis steyermarki, neotropical fish of the anostomid family found in French Guiana, using starch-gel protein electrophoresis. Fifteen enzymatic proteins representing twenty loci have been screened. This revealed marked genetic differences between these species, which are hardly morphologically discernable and indicates they are reproductively isolated. Diagnostic loci for unequivocal species identification have been found and will allow establishment of pure breeding stocks for planned aquaculture of Leporinus species.