The highest production of extracellular cellulases (Filter paper activity, Carboxymethyl cellulase and β-glucosidase) by Trichoderma harzianum was achieved on delignified wheat straw. Glycerol at a concentration of one per cent strongly repressed the formation of all the cellulolytic components. Addition of cyclic AMP did not relieve the repression caused by glycerol but lowered the level of existing cellulase enzymes.