Fed-batch culture Mas performed to minimise growth inhibition of Chlamydomonas reinhardtii by acetate in heterotropic culture. Although the cell concentration obtained was higher than in batch cultures, it was eventually constrained by high sodium concentrations, which accumulated in the fermenter. The use of a continuos-flow, hollow-fibre cell-recycle system (HFCRS) using a membrane with a 0.2 mu m pore size was successfully employed to overcome this problem. The HFCRS was run with no cell bleed to obtain a cell concentration of 9 g/l at a dilution rate of 0.1/h and a feed acetate concentration of 1.7 g/l. However the cell growth yield under these conditions was poor: Using the same dilution rate and feed acetate concentration a superior cell productivity of 0.083 g/l/h was achieved at a bleed ratio of 0.5. An unstructured mathematical model, incorporating the maintenance coefficient, was derived for the HFCRS. The model gave goon agreement with experimental results, particularly at high bleed ratios, and predicted that higher cell concentration and productivity could be obtained using the HFCRS with a higher feed acetate concentration and dilution rate. The results demonstrate that the HFCRS is well suited to obtaining high concentrations of microalgae grown heterotrophically on inhibitory substrates. Copyright (C) 1996 Elsevier Science Ltd.