A uridine auxotrophic mutant (pyrG) of Trichoderma reesei was successfully transformed to constitutively produce chitinase using a construct containing an Aphanocladium album cDNA chitinase gene fused in frame to the glyceraldehyde-3-phosphate dehydrogenase promoter of Aspergillus nidulans. The transformation frequency was high (897-1,421 transformants mu g(-1) DNA) and up to 70% of the stable transformants exhibited extracellular chitinase activity Molecular characterization of the transformed strains demonstrated that the chitinase gene had integrated into the T. reesei genome. The transformed T. reesei expressed an extracellular enzyme at a specific activity 6.5-fold higher than the intracellular level at peak production, and the enzyme had a calculated molecular mass of 52 kDa.