Chitinase B from Serratia marcescens 2170 is one of the processive chitinases, and it has a linear path of aromatic amino acid residues on the surface and in the catalytic cleft. There are four surface-exposed residues lined-up towards the cleft, Y481, W479, W252, and Y240. The substitution of these residues with alanine causes a decrease in both the extent of the substrate binding and the hydrolytic activity (Katouno et al., 2004). Here, we examine the three mutants without losing the substrate-binding ability, Y240W, Y481W, and Y240W/Y481W. These mutants were prepared for a detailed analysis of the functions of Y240 and Y481, which showed a lower contribution to substrate binding than W479 and W252. The parameters for the binding of the three mutants to crystalline β-chitin were similar to those for the wild type. The hydrolytic activity of Y240W and Y240W/Y481W against crystalline β-chitin was significantly decreased. However, the hydrolytic activity of Y481W was similar to that of the wild type, indicating some differences in the roles of Y240 and Y481 during the processive degradation of crystalline β-chitin. Taken together with the previous results, it was suggested that while Y240 and Y481 were required for the substrate binding, Y240 had additional roles in the processive degradation of crystalline β-chitin, possibly in guiding a chitin chain into the catalytic cleft.