The activity of xenobiotic-degrading microorganisms is generally high in a biphasic aqueous/organic system, Therefore, the influence of interfacial area variation on kinetic parameters of Candida sp. growing on ethyl butyrate was evaluated. Interfacial areas of both aseptic and cultured biphasic systems were utilized. Substrate transport measurements in aseptic system (where the interface varied with the organic-phase fraction and agitator speed) showed that the substrate concentration in the aqueous phase was constant at different agitation speeds and decreased as the organic phase increased. Kinetic measurements of the cultured system showed that kinetic parameters vary as functions of their respective aseptic interfacial areas. Higher mu(max) and K-i and lower K-s values were obtained with larger interfacial areas. Measurements of the cultured system showed that the interfacial area increased as the biomass increased, and that about 50% of the biomass was attached to the interface as an interfacial biofilm at the end of the culture. Results suggest that the growth and selection of xenobiotic-degrading microorganisms in a biphasic aqueous/organic system should be evaluated mainly on the basis of the activity of adhering biomass (forming a biofilm) at the interfacial area rather than on substrate transport to the aqueous phase