SUMMARY:A filamentous mould was cultured by the continuous flow method in which medium is supplied at a constant rate and the culture volume is kept constant. Flow rates up to 0·1 culture volumes/hr were used. The mycelial dry weight concentration and the yield of mycelium/g of carbon source used were equal to or slightly greater than the maximum obtained in batch culture. With glucose concentrations up to 80 g/1. at a flow rate of 0·05 culture volumes/hr, about 45% of the substrate carbon was converted into mycelial carbon and the remainder oxidized to CO2.With unlimited amounts of all nutrients available growth of the mould followed the exponential law, as does bacterial growth, and therefore the mould had a constant doubling time.The oxygen demand of the mould as function of growth rate was determined.Conditions were found under which the rate of penicillin production/g of mycelium remained at its maximum value for 1000 h