The basidiomyceteCoprinopsis cinereaproduces 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 deter-mined the crystal structures of CcCel6C unbound and in complex with p-nitrophenyl b-d-cellotrioside and cellobiose.