The basidiomycete Coprinopsis cinerea produces the glycoside hydrolase family 6 enzyme CcCel6C at low and constitutive levels. CcCel6C exhibits unusual cellobiohydrolase activity; it hydrolyses carboxymethyl cellulose, which is a poor substrate for typical cellobiohydrolases. Here, we determined the crystal structures of CcCel6C unbound and in complex with p-nitrophenyl beta-D-cellotrioside and cellobiose. CcCel6C consists of a distorted seven-stranded beta/alpha barrel and has an enclosed tunnel, which is observed in other cellobiohydrolases from ascomecetes Hypocrea jecorina (HjeCel6A) and Humicola insolens (HinCel6A). In HjeCel6A and HinCel6A, ligand binding produces a conformational change that narrows this tunnel. In contrast, the tunnel remains wide in CcCel6C and the conformational change appears to be less favourable than in HjeCel6A and HinCel6A. The ligand binding cleft for subsite -3 of CcCel6C is also wide and is rather similar to that of endoglucanase. These results suggest that the open tunnel and the wide cleft are suitable for the hydrolysis of carboxymethyl cellulose.