Chitobiase from Aeromonas salmonicida YA7-625 was purified from culture broth through ammonium sulfate precipitation, chitin affinity adsorption, hydroxylapatite column chromatography and gel filtration, with 47.2% yield and 31.5 fold purity. The molecular weight of purified chitobiase was 15,000 daltons, and the chitobiase showes maximum activity at the condition of at 40℃ and pH 6.0. The effects of various metal ions and inhibitors show thatcystein, glutamic acid, serine, tryptophan, and tyrosine residues seem to be cencerned in chitobiase activity.