Abstract: Lactate dehydrogenase isozyme of 3 species of catfish,I.e.Clarias fuscus,C.macrocephalus,C.lazera and their F1 hybrids,C.macrocephalus×C.fuscus,C.fuscus×C.lazera and C.macrocephalus×C.lazera,were studied by polyacrylamide gel electrophoresis.These catfish all show the same electrophoretic mobility of LDH-A4 isozyme.The B4 isozyme of C.lazera has a slower mobility than that of other two species.C.fuscus and C.macrocephalus shows 5 bands encoded by A,B loci and C.lazera shows 3.Liver homogenates reveal LDH-C isozyme that migrated cathodal to the A4 isozyme.This isozyme has the same mobility in all samples.In C.macrocephalus and C.lazera,the brain tissue also shows the cathodal isozyme.The F1 hybrids posses some LDH isozymes that encoded by the gene loci from their parents.In C.macrocephalus×C.fuscus,all five bands are the same as that in its parents.In other two hybrids,the isozyme shows heterozygosity.According to the isozyme phenotypes we discussed evolution,inheritance and cross breeding problems in these catfish.