A new, thermostable superoxide dismutase (SOD) from Bacillus licheniformis M20, isolated from Bulgarian mineral springs, was purified 11-fold with 11% recovery of activity. From native PAGE and SDS-PAGE, the enzyme was composed of two subunits of 21.5 kDa each. The SOD was inhibited only by NaN3, which suggested that this SOD is of the manganese superoxide dismutase type. The purified enzyme had maximum activity at pH 8 and 55 degrees C. The half-life of the SOD was 10 min at 95 degrees C.