Bacillus horikoshii which produces an extracellular alkaline protease, was isolated from the body fluid of an unique Korean polychaeta (Periserrula leucophryna) living in the tidal mud flats of Kwangwha island in the Korean West Sea (Yellow Sea). Maximum enzyme activity was achieved when the bacterium was grown in soybean meal (1.5%, w/v) and casein (1%, w/v) at pH 9.0 and 34 degreesC over 16-18 h incubation period. The enzyme has an optimum pH of around 9 and maintained its stability over a broad pH range between 5.5 and 12. Its optimum temperature is around 45 degreesC, and exhibited a stability of up to 50 degreesC. The enzyme activity was strongly inhibited by PMSF, suggesting that it belongs to the family of serine proteases. (C) 2002 Elsevier Science Ltd. All rights reserved.