A study has been made of the growth of the microalga Scenedesmus obliquus in a continuous photobioreactor operating as a chemostat. The influence of light intensity was analysed at different rates of dilution, under limited as well as saturating intensities of illumination. In order to achieve efficient removal of phosphorus (P) from the liquid medium, it is advisable to operate at a high light intensity with the working dilution rate near the critical maximum dilution rate. At all light intensities, conditions of maximum P removal differed from those of maximum biomass productivity, the former being displaced at higher dilution rates. The removal of P from the medium was aided by surface adsorption to the cells and the culture vessel.