Plant expansin proteins induce plant cell wall extension and have the ability to extend and disrupt cellulose. In addition, these proteins show synergistic activity with cellulases during cellulose hydrolysis. BsEXLX1 originating from Bacillus subtilis is a structural homolog of a β-expansin produced by Zea mays (ZmEXPB1). The Langmuir isotherm for binding of BsEXLX1 to microcrystalline cellulose (i.e., Avicel) revealed that the equilibrium binding constant of BsEXLX1 to Avicel was similar to those of other Type A surface-binding carbohydrate-binding modules (CBMs) to microcrystalline cellulose, and the maximum number of binding sites on Avicel for BsEXLX1 was also comparable to those on microcrystalline cellulose for other Type A CBMs. BsEXLX1 did not bind to cellooligosaccharides, which is consistent with the typical binding behavior of Type A CBMs. The preferential binding pattern of a plant expansin, ZmEXPB1, to xylan, compared to cellulose was not exhibited by BsEXLX1. In addition, the binding capacities of cellulose and xylan for BsEXLX1 were much lower than those for CtCBD3. Biotechnol. Bioeng. 2013; 110: 401–407. © 2012 Wiley Periodicals, Inc.