Studies on the enzymes of the cellulase complex of the thermophilic fungus Humicola grisea var. thermoidea are described. A genomic library was constructed in the phage vector EMBL 4, and from this library two clones were isolated using as a probe the cloned cbh-1 (exoglucanase, EC 3.2.1.91) gene of Phanerochaete chrysosporium, a cellulolytic basidiomycete fungus. These clones were analysed by restriction mapping and Southern blotting, and one of them (lambda 3) was sub-cloned into the M13 phage vectors mp18 and mp19. The gene sequence was determined by the dideoxy chain-termination method. Sequence comparison with the equivalent genes from P. chrysosporium and Trichoderma reesei was made: in terms of primary sequence there is about 60% homology between the three species. Secondary structure prediction of the H. grisea sequence was also computed.