The major extracellular proteases of Aeromonas hydrophila NRC 505 and ATCC 7966 are a thermostable metallo-protease (TSMP) and a thermolabile serine-protease (TLSP), respectively. The purified enzymes differed in sensitivity to heating at 56°C for 30 min, in response to the inhibitors EDTA and phenylmethylsulphonyl fluoride (PMSF), and were antigenically distinct. When crude extracellular products (ECP) of 47 strains of A. hydrophila were screened, 27 strains produced both TSMP and TLSP. TSMP was the only extracellular protease produced by 19 strains, whereas only A. hydrophila ATCC 7966 produced TLSP as its sole protease. This distribution of protease enzymes among strains explains conflicting reports from studies in which single strains of A. hydrophila were examined. The TLSP of A. hydrophila was similar to the protease of A. salmonicida. Either or both TSMP and TLSP were produced by some strains of A. sobria and A. caviae.