Abstract 1. 1. The present paper reports some aspects of the isozymes of LDH, MDH and GPI in fish. 2. 2. In Petromyzontiformes LDH is encoded by a single Ldh-A gene locus. In Myxiniformes and in most vertebrates LDH is encoded by two gene loci, A and B. A third Ldh-C locus is characteristic of the bony fishes Actinopterygii. 3. 3. In fish the MDH isozymes are generally encoded by three gene loci Mdh-M, Mdh-A and Mdh-B. 4. 4. In most diploid bony fish the GPI is controlled by two independent gene loci Gpi-A and Gpi-B. 5. 5. The relationships of isozymes with evolution of vertebrates, tissual specificity, ontogenetic changes, with physiological and metabolic roles are discussed.