The possibility of using heterotrophic chemostat culture to achieve relatively high cell densities of Chlamydomonas reinhardtii by avoiding the inhibition of growth by the substrate, acetate was investigated Continuous culture gave significant improvement in cell dry weight concentration and productivity compared to batch culture. It is postulated that this was due to low acetate concentration in the continuous culture. The highest cell concentration obtained was 1.5 g/litre at a feed acetate concentration of 3.4 g/litre. At acetate concentrations up to this level, the modified Monod model successfully predicted the steady-state cell concentration and cellular productivity. However no further increase in cell concentration was achieved, despite the use of higher feed acetate concentrations (i.e. 5.1 g/litre), in which, steady-state acetate concentrations were below the inhibition level (i.e. 0.4 g/litre). This inhibitory effect was probably due to sodium rather than to acetate per se in the cultures.