Circadian oscillations of penicillin productivity with a period of 22 +/- 2 h have been observed in carbon-limited batch fermentations of Penicillium chrysogenum. The specific penicillin production rate oscillated with an amplitude of 20 to 100% of its mean value, depending on the growth rate of the active (respiring and producting) biomass. In spite of this, the penicillin concentration increased almost linearly if the optimum growth rate of the active biomass for maximum penicillin productivity was maintained using microprocessor control. This apparently inconsistent behaviour of the fungus is discussed on the basis of chaos theory.