The production of extracellular catalase in a submerged culture by a number of biochemical mutants has been evaluated. Eight of these mutants showed increased extracellular catalase, the level of which ranged widely in individual cases from 44% to over 94% in comparison with the parental strain. Studies of the relationship between a criterion of selection and the frequency of mutation showed that the highest frequency of positive mutations (15.8% and 24.2%) was obtained with respect to mutants resistant to ethidium bromide (1 mmol/l) and sodium gluconate (45%) respectively. The time course of growth and enzyme production by the most active mutant, AM-20, showed extra- and intracellular catalase activities increasing about 2- and 2.6-fold respectively, compared with the parental strain.