SummaryIsoenzyme patterns were studied in local populations of the carp (Cyprinus carpio) and the bream (Sarotherodon mossambicus) of the Cyprinid family. and in the trout (Sabno giardneri) of the family Salmonidae. Homogenates of heart muscle extracts were used in the identification of PGI. NADP‐IDH. PGM and 6‐PGD.Polymorphisms found and gene frequencies obtained are discussed separately for each enzyme and species or population.In the bream, variation was found only at the PGI locus. In the carp only the PGM locus was polymorphic whereas both PGM and IDH showed variation in the trout. The variation at the PGM locus in the trout cannot support the three locus model suggested elsewhere and clearly indicates a single locus for the mo‐nomeric enzyme.The large variety of NADP‐IDH types in trout not only illucidates the complexity of this locus but confirms a disomic mode of inheritance. Genetic differences in the trout populations could be related to possible advantages for management p