The removal of phosphorus and nitrogen by the freshwater alga Scenedesmus obliquus, cultured in urban wastewater, previously submitted to secondary sewage treatment, was studied under different conditions of stirring and temperature. In all cases, the amount of NH3 lost, as well as biomass productivity and its biochemical composition, were evaluated. The specific growth rates proved greatest in the stirred cultures, the highest mu value being 0.0438 h(-1) at 30 degrees C. The stirring increased biomass productivity (P-B) in the linear growth phase after exponential growth, with the optimum appearing at 25 degrees C. For the temperatures studied stirring was not necessary to provide the highest percentage of P elimination (%P-max), but did reduce the time needed to reach that percentage (t(max)). The highest %P-max value, 98%, within the shortest time period, t(max) = 94.33 h, was found in the culture with stirring at 25 degrees C. Ammonium removal was determined by two factors - the consumption of ammonium for growth and elimination by desorption as ammonia. The highest percentage of ammonium removal (%N-max), 100%, resulted at the final culture time (t(f)) of 188.33 h, in the stirred culture at 25 degrees C. The biochemical composition of the biomass gave the normal values for this microalga reported by other authors. The protein content was notably low, around 11.8% by weight, and the polyunsaturated-fatty-acid content was high. The N:P ratio of the culture medium was 12.9. Finally, we proposed a dilution factor for the treated wastewater (f) to be dumped in order to regulate operation conditions and time for an optimal removal of nitrogen and phosphorus.