With the object of obtaining information about the suitability of filamentous fungi for production of heterogeneous mammalian proteins, we have characterized the normal glycosylation pattern of CBH I, the major glycoprotein secreted by Trichoderma reesei. The protein–bound glycans of CBH I were studied using a variety of separation techniques to analyze the products of specific glycosidase digestions and chemical degradation procedures. CBH I contains both O–glycosidic and N–glycosidic glycans. The O–glycosidic glycans consist of one to four hexose residues which are mostly mannose. The structures of the N–glycosidic glycans are (Man)5(GlcNAc)2 and (Man)9(GlcNAc)2. Thus it may be possible to use T. reesei as a host for the production of heterologous glycoproteins of animal origin, and to take advantage of its remarkable secretory capacity.