The heterotrophic growth of microalgae has been proposed as a potential mode of culture to achieve high cell densities on a large scale. In this paper, the heterotrophic growth of the green microalga, Chlamydomonas reinhardtii, on acetate as sole energy and carbon source in batch and single-stage continuous cultures is reported. No inhibition by acetate was observed at low concentrations (less than or equal to 0.4 g/litre), in which case, cell growth was well described by the Monod model. However, above this acetate concentration, growth was progressively inhibited. Inhibition was observed as a decrease in both the specific growth rate and the observed growth yield. The former was well modelled using the Haldane model. A model based on constant q(m) was proposed to describe the latter phenomenon. Whereas it predicted the trend of falling observed cell growth yield with initial acetate concentration in batch cultures, there was significant lack of fit between simulated and experimental data. In continuous cultures, a steady state cell concentration of 0.4 g/litre and maximum cell productivity of 0.02 g/litre/h was obtained for an acetate feed concentration of 0.85 g/litre. This compares well with values achieved by photosynthetic mass culture systems.