Pseudomonas oleovorans produces significant amounts of surfactants when grown on n-alkanes in two-liquid phase media. We determined the stability and droplet size distribution of emulsions formed by these biosurfactants in cell-free decane-water mixtures at different stirring speeds in a tank reactor. Above 1,500 rpm in the presence of 1 g l(aq)(-1) biosurfactant, stable emulsions were fonned with Sauter mean droplet diameters between 10-13 mu m.Maximal volumetric growth rates were determined in two-liquid phase P. oleovorans cultures grown on n-decane as the sole carbon source. In these cultures containing either only the surfactants produced by the bacteria during the cultivations or additional surfactant added beforehand, limited decane transfer from the apolar solvent phase to the cells led to linear growth.When stirred continuously at 500 rpm, the maximum volumetric growth rate under decane limitation, an indicator of the maximum decane transfer rate, was 0.7 g l(aq)(-1) h(-1). When the culture was emulsified at 2,500 rpm during the initial 12 h and then stirred at 500 rpm and an addition of 1 g l(aq)(-1) surfactant isolated from previous cultures was added, maximal volumetric growth rates decreased to 0.4 and 0.25 g l(aq)(-1) h(-1), respectively.We concluded that efforts to maintain surfactant concentrations at low levels and to control surfactant composition should improve mass transfer of apolar substrates in two-liquid phase bioprocesses.