Protoplasts from a benomyl resistant Trichoderma reesei mutant were heat inactivated at 60 degrees C for 8 min and fused with viable protoplasts from an osmosensitive, non-sporulating T. reesei strain. Fusants recovered on 50 mu g/ml benomyl containing potato dextrose agar plates grew and sporulated well. Cellulolytic enzyme activities produced in liquid culture by selected fusants were higher than those produced by parental strains.