A flake-chitin degrading marine bacterium was isolated and identified as Aeromonas hydrophila. This strain secreted five chitinases and an beta-N-acetylglucosaminidase. The main chitinase (Chi-A) was purified and characterized. The optimum pH of Chi-A was 5-8, and the activity was inhibited by Hg2+ and Fe3+. Chi-A was different from chitinases of other Aeromonas species with respect to molecular weight (62,000) and insensitivity to monoiodoacetate. The amino-terminal amino acid sequence showed extensive similarity with chitinases from Gram-negative bacteria.