A crude cellulase preparation from Aspergillus niger was used to depolymerize chitosan. The depolymerization process was followed by measuring the apparent viscosity and the intrinsic viscosity. The optimum conditions for enzymatic hydrolysis were investigated. On the selected optimum conditions (pH 5.0, temperature 50 degrees C, and an enzyme to substrate ratio of 1:5), chitosan was hydrolyzed for 1, 4, 8, and 24 h, its viscosity-average molecular weights were 3.49 x 10(4), 1.18 x 10(4), 5.83 x 10(3), and 1.13 x 10(3), respectively. Compared with chitosan having viscosity-average molecular weight of 5.18 x 10(5) before enzymatic hydrolysis, the crude cellulase preparation had rather apparent effect on depolymerization of chitosan. Through the comparison of different origin of cellulases, the prepared cellulase has good ability of enzymatic hydrolysis. The reproducibility and reversibility for enzymatic hydrolysis was appraised. The data are of value for the production of low-molecular weight chitosans and chitooligomers of medical and biotechnological interest.