A strain improvement program was developed to increase extracellular phytase (E.C. 3.1.3.8.) production by Aspergillus niger (syn. A. ficuum) NRRL 3135. Ultraviolet (UV) radiation was used as the mutagen and resistance to 50 mu g/ml of hygromycin B as the selection method. Mutant 2DE, the product of two UV treatments, had phytase (PhytA) activity at pH 5.0 in the extracellular filtrate that was 3.3-fold higher than the wild-type activity. The activity of the non-specific acid phosphatase with a pH optimum of 6.0 (Pase) was one-fifth the activity of the wild type and the non-specific acid phosphatase with a pH optimum of 2.5 (PhytB) was not significantly different from the wild type. The mutant and wild-type PhytA, PhytB and Pase responsed similarly in inhibition studies. However, the wild-type enzymes were inhibited more by 1 mM sodium fluoride and 1 mM phosphomycin. PhytA production by the mutant was repressed 60% by inorganic phosphate concentrations of 0.006% (wt/vol) or above. The mutant had an extracellular protein concentration 3.2-fold higher than the wild type, which correlated with its higher phytase activity at pH 5.0, but not with phytase activity at pH 2.5 and acid phosphatase activities. The isolate may be a phytase catalytic mutant, as well as, on overproducer of phytase. In addition, a mutant with an apparent lack of activity of all three acid phosphatases was isolated.