We have measured the rate of (-)-menthol production by whole cells of Bacillus subtilis in a stirred aqueous emulsion of menthyl acetate. With excess cells, the rate is proportional to the organic phase volume, but is the same whether this is pure (-)-menthyl acetate substrate or the racemate. This result is expected if substrate uptake is by direct contact with the interface, but not if mass transfer into the aqueous phase is. limiting. This suggests a general approach for distinguishing these mechanisms, which usually give the same steady-state kinetic behaviour. Addition of 2-propanol, which increases the solubility of menthyl acetate, has little effect an the rate, supporting the conclusion that substrate uptake is by interfacial contact.