We have studied the expression of a bovine chymosin cDNA in Trichoderma reesei to test the ability of this novel fungal host vector system to express and secrete heterologous gene products. Four different expression plasmids were constructed to determine the optimum manner to fuse the chymosin cDNA to the promoter and the terminator of the major T. reesei cellulase gene, cellobiohydrolase I (cbh1). All four constructions, when transformed into Trichoderma, determined the secretion of a polypeptide corresponding in size to prochymosin and reacting with antichymosin antiserum. This polypeptide had a molecular weight indistinguishable from chymosin and showed milk clotting activity. In preliminary fermentation studies, one transformant secreted as much as 40 mg/1 of active chymosin.