The extracellular proteases of Aeromonas hydrophila B51 were stable on heating (56 degrees C) and on storage at 4 degrees C or -20 degrees C. Inhibitor studies showed that 72% of the total activity was inhibited by EDTA (a metalloprotease inhibitor) and 26% was inhibited by phenylmethanesulphonyl fluoride (a serine protease inhibitor). Analytical isoelectric focussing revealed the presence of 33 proteins in the crude extracellular products. Using a casein overlay technique three separate zones of proteolytic activity were detected: a zone with pI 6.5-6.8, formed of two closely focussed bands (possibly isomers of the same protease) and completely inhibited by EDTA; a single band with pI 7.0, which was inhibited by EDTA; and a diffuse zone with pI 8.3-8.5, which was only partially inhibited by EDTA. It is concluded that the serine protease activity focussed in this latter zone. These results indicate the presence of at least four, and possibly five proteases. Our results differ substantially from those reported by other workers using different isolates and it is suggested that significant differences in the character of extracellular products and extracellular proteases exist between different isolates of A. hydrophila.