Scenedesmus obliquus cells were immobilized by adsorption in preformed polyurethane and polyvinyl foams as well as by entrapment using urethane prepolymer. Adsorption on polymeric foams appeared to be the most convenient immobilization method, mainly when cells were initially adsorbed in a nitrogen-free medium. Nitrogen starvation of cells appeared to enhance their adsorption to the polymeric foams. The net O-2 evolution activity and nitrate removal capacity of free-living and polyvinyl-adsorbed cells were similar, indicating that immobilization did not significantly affect the physiology of microalgae. The N removal capacity of polyvinyl-adsorbed cells was studied in batch and in a lab-scale continuous-flow bioreactor. Nitrogen starvation greatly increased the N uptake rate of immobilized cells. The effects of illumination (intensity and photoperiod), residence time, and CO2 supply were investigated with respect to the N removal efficiency of the bioreactor. It was concluded that S. obliquus cells adsorbed in hydrophilic, commercially available polymeric foams are of potential value for biologic N depollution of drinking waters contaminated with nitrogenous fertilizers.