1. 1. Electrophoretic analyses of the lactate dehydrogenase isozyme patterns of 23 species of tropical fishes belonging to the orders Cypriniformes, Siluriformes and Perciformes indicates that at least two LDH loci—Ldh-A and Ldh-B are active. In the Perciformes species, in addition to these loci, the Ldh-C is also present, and restricted in its expression to the eye and brain. 2. 2. Genetic variants were detected in the Ldh-A and Ldh-B loci of Leporinus friderici and in the Ldh-B locus of Schizodon borelli Cypriniformes. 3. 3. An increase in the occurrence of the Ldh-BB′ and Ldh-B′B′ allotypes of Leporinus friderici was observed along the successive collects that started in 1980. This change in phenotypic frequency suggests that this polymorphism could be related to environmental conditions.