Extraction and stripping kinetics of lactic acid in extractive fermentation were studied using tri-n-octylmethylammonium chloride, a quaternary ammonium salt, as an extractant and oleyl alcohol as a diluent. The kinetic analysis was performed through simulated time course studies of lactate concentration in both the extraction and stripping phases. Dependences of extraction rate on initial lactate and extractant concentrations and those of stripping rate on initial chloride and extractant-lactate complex concentrations were examined. Because diffusion through the organic film was a rate-controlling step, the experimental results of both extraction and stripping could be explained well by a simple equation.