Among 162 strains of the genus Fusarium tested, 22 strains, mainly from F. solani and F. splendens, formed halos on chitosan-containing agar medium. Chitosanase secreted in the culture by the most effective producer, F. solani f. sp. phaseoli SUF386, was further investigated. N-Acetylglucosamine (GlcNAc) used as a carbon source was most effective for production of chitosanase. Chitosan, a substrate for chitosanase, inhibited cell growth completely and abolished production of chitosanase when used as a carbon source in the liquid medium. Chitosanase purified from the culture filtrate had a molecular mass of 36 kDa, and showed a maximum activity at pH 5.6 and 40°C. The enzyme catalyzed the hydrolysis of chitopentaose, chitosan (70% and 100% deacetylation), and glycolchitosan, but showed little activity toward chitobiose, chitotriose, chitotetraose, glycol chitin, and carboxymethyl cellulose. A rapid reduction in the viscosity of chitosan solutions suggested that the enzyme catalyzed an endo-type cleavage reaction.