Cel9B, an endocellulase produced by Thermobifida fusca YX, contains a number of structural domains, including carbohydrate binding modules 2 and 4 (CBM2 and CBM4), a fibronectin-like (Fn3) domain, an Eset domain (an Ig-like domain that may play a role in enzyme folding), a catalytic domain, and a fibronectin-like (Fn3) domain. To elucidate the roles of these domains with respect to Cel9B function, a series of truncation mutants were designed and examined for their binding properties and activities on different substrates. Different binding properties of CBM2 and 4 with a variety of substrates distinguish important roles for these domains and provide insight as to how distinct domains interact with each other during substrate degradation. The results of this study implicate the collective roles of the non-catalytic domains with respect to Cel9B function, and in turn, this information can be incorporated into protein engineering strategies for improved biomass conversion.