The aqueous substrate and product toxicity thresholds in the microbial biotransformation of (-)-trans-carveol to the fragrance/flavor compound (R)-(-)-carvone by Rhodococcus erythropolis were determined. Above aqueous phase concentrations of approx. 500 mg carveol/l and 200-600 mg carvone/l, the biotransformation activity of the biocatalyst was inhibited. This biotransformation was undertaken in a single aqueous phase 3 reactor in which a total of 5 ml carveol (mixture of isomers) was added before the biotransformation rate decreased significantly. The carvone volumetric productivity was 31 mg/lh. Although the growth of the organism post-exposure was not affected, dramatic morphological changes in response to the accumulation of the inhibitory substrate and product were observed.