The effect of surfactants on aeration performance in stirred tank reactors (STR) at high rates of foaming is studied. The volumetric oxygen transfer coefficient (k(L)a) and foaming activity estimated as foaming height (H-f) were determined. Biotechnology of lipopeptide biosurfactants from aerobic organisms, e.g., Bacillus subtilis were addressed. Using model solutions of known foam-generating properties, high-molecular weight surfactin and low-molecular weight sodium dodecyl sulphate (SDS), as well as impellers of different types, with flat and fluid-foil blades, clues on the concentration dependence of STR oxygen transfer and foaming as well as options for foam reduction in the presence of biosurfactant were sought. In response to a two-fold decrease of surface tension by surfactin, k(L)a values decreased up to 30% but remained within the range expected for the mixing system in water; the experiments with SDS showing stronger dependence on surfactant concentration and surface tension. Mixing of surfactant media by a standard six-blade disc turbine (RT) imposed rate limitations on gassing. A low-shear impeller Narcissus (NS) could be used to avoid bulk foam outflow, while preserving k(L)a values that remained unchanged. The 'power per unit volume' correlation of k(L)a in stirred tanks is tested in the presence of surfactin.