Abstract: Five isozymes (EST, ME, AST, LDH and MDH) in six tissues (muscle, heart, liver, kidney, brain and spleen) of Cheilinus undulatus were studied by using vertical polyacrylamide gel gradient electrophoresis. The results showed that the five isozymes presented tissues specificity to some degrees. The number of EST bands was 2, which were encoded by 2 loci. LDH existed in all tissues studied. The number of LDH bands was 9, which were encoded by 3 loci, and the polymorphism was strong. Two types of MDH (m-MDH and s-MDH) were found and also existed in the six tissues. The number of ME bands was 2. ME in liver were rich, but it was not distinct between m-ME and s-ME. There was no AST in all six tissues.