Catalase activities are determined on the liver extracts of the various species of teleost, and optimum pH of each species are shown graphycally. Optimum pH of liver catalase of the many saline water fishes are found to be 6.9-7.2, but of the several fresh water fishes are found at low optimum pH, 6.1-6.2. These low values of opt. pH seem to be characteristic of the fresh water fishes.