In this work the biotransformation of alpha-pinene oxide to isonovalal using resting cells of Pseudomonas fluorescens NCIMB 11671 was evaluated in a membrane bioreactor for biotransformations (MBB). Since the membrane area required to obtain optimum productivities was calculated to be very large (1,000 m2 m(-3)), and not possible to fit into the laboratory reactor used, we initially evaluated performance with lower membrane areas (71 m2 m(-3)) in a batch system with both the substrate and product in the organic phase. This resulted in low productivities due to mass transfer limitations, so an optimum feeding rate of 0.1 g alpha-pinene oxide h(-1) g(cells) (-1) added directly to the reactor contents was determined in batch culture to minimise inhibition. The MBB was then operated continuously for the production of isonovalal, and a final concentration of 108 g l(-1) was obtained in the organic reservoir after nearly 400 h of operation (0.32 g-isonovalal l(-1) h(-1)), and the reaction was found not to be mass transfer limited. Finally, the relative viability of the cells was measured using fluorescent probes, and their half-life was found to be almost 2 months, confirming the ability of the MBB to facilitate biotransformations with inhibitory substrates and products.