Lactate dehydrogenase isozymes in heart, kidney, liver and skeletal muscle of 15 species of vertebrate animals belonging to 5 classes were separated by cellulose acetate electrophoresis and the levels of them were measured and compared with each other. Lactate dehydrogenase isozyme patterns were different from each other among animal species and among tissues. The activity of LDH$_{5}$ was superior in anaerobic tissues such as liver and skeletal muscle, and the activity of LDH$_{1}$ was superior in aerobic tissues such as heart and kidney. The level of LDH of vertebrate animals of the 5 classes has found approximatry increasing in the following order: Pisces>Amphibia>Reptelia<Aves<Mammalia.