The use of a fed-batch cultivation of the fungus Trichoderma reesei (C30) allows cellulase [see 1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4] production to occur under optimum conditions, and results in extremely high enzyme titres and productivities. Enzyme levels of 26 U ml−1 at productivities >130 U l−1 h−1 have been achieved. These results are compared with the values obtained in two-stage continuous cultivation of the organism at optimum pH and temperature.