Chitosan of 80% degree of deacetylation was depolymerised by the cellulase of Aspergillus nigerto prepare chitooligosaccharides. MALDI-TOF MS analysis of the enzymatic hydrolyzates indicated that the products were mainly chitooligosaccharides with degree of polymerization (DP) in the 3–11 range. By fractionation of the hydrolyzates with acetone–water, products of DP below 6 and DP above 6 were separated in good yield.