The objective of this work was to improve the production of hexanoic acid by the anaerobic rumen bacterium, Megasphaera elsdenii, using product removal and immobilised cell approaches. Hexanoic acid, the major product of glucose metabolism by M. elsdenii strain ATCC25940, was produced at concentrations of 2-3 g dm(-3) in stirred batch cultures. With pH controlled manually at 7, maximum concentrations of hexanoic acid increased to 6-8 g dm(-3) with yields (g product per g glucose used) of approximately 30%. When an anion exchange resin, Amberlite IRA 400, was added during early stages of culture to minimise product inhibition, growth was not impaired and cell lysis, which was commonly seen during the stationary phase in control fermentations, was prevented. The presence of resin in pH-controlled, stirred batch fermentations increased the rate of glucose consumption and doubled hexanoic acid productivity : the equivalent of 11 g dm(-3) of hexanoic acid was made with an estimated yield of up to 39%. Cells were immobilised successfully in rc-carrageenan and, when cell densities in inocula were sufficiently high, rates of glucose consumption and product formation were similar to free cells. Including resin in cultures of immobilised cells had effects similar to those above. Using a fed-batch mode with immobilised cells cultured in the presence of resin further increased final concentrations of hexanoic acid (up to 19 g dm(-3)) but yields were lower (20-30%) and productivity did not increase. These results show that production of volatile fatty acids can be improved significantly by product stripping onto an anion exchange resin.