The purified endochitosanase(Mw 41 kDa) from bacterium Bacillus cereus D-11 hydrolyzed chitooligomers $(GlcN)_{5-7}$ into chitobiose, chitotriose, and chitotetraose as the final products. The minimal size of the oligosaccharides for enzymatic hydrolysis was a pentamer. To further investigate the cleavage pattern of this enzyme, chitooligosaccharide alcohols were prepared as substrates and the end products of hydrolysis were analyzed by TLC and HPLC. The chitosanase split $(GlcN)_4GlcNOH$ into $(GlcN)_3+(GlcN)_1GlcNOH$, and $(GlcN)_5GIcNOH$ into $(GlcN)_4+(GlcN)_1GlcNOH$ and $(GlcN)_3+(GlcN)_2GlcNOH$. The heptamer $(GlcN)_6GlcNOH$ was split into $(GlcN)_5$ [thereafter hydrolyzed again into $(GlcN_3+(GlcN)2]+(GlcN)_1GlcNOH$, $(GlcN)_4+(GlcN)_2GlcNOH$, and $(GlcN)_3+(GlcN)_3GlcNOH$, whereas $(GlcN)_{1-3}GlcNOH$ was not hydrolyzed. The monomers GlcN and GIcNOH were never detected from the enzyme reaction. These results suggest that D-11 chitosanase recognizes three glucosamine residues in the minus position and simultaneously two residues in the plus position from the cleavage point.