The chitosanase of Bacillus cereus NTU-FC-4 was used for the production of high degree polymerized (DP) chitooligosaccharides. In the aqueous system, the chitosanase was found to be able to catalyze a reaction to yield small amount of octamer from a mixture consisting of dimer, trimer. tetramer, pentamer, hexamer and heptamer, possibly through the transglycosylation reaction. To enhance the possible transglycosylation reaction, the process was carried out in the reversed micellar microreactors formed by AOT (sodium bis-2-(ethylhexyl) sulfosuccinate) in isooctane, and the formation of high DP chitooligosaccharides was significantly increased. It was found that the water content in the reverse micelles was an important factor affecting the enzymatic reaction. When the molar ratio of water to surfactant (WO) was 11.86, the reaction yielded the highest amount of heptamer, octamer, and nonamer. Hydrolysis of pure chitooligosaccharides by the chitosanase revealed that pentamer and hexamer might be the main gylcosyl acceptors during enzymatic reaction. (c) 2007 Elsevier Ltd. All rights reserved.