Semicontinuous cultures of Scenedesmus obliquus in artificial wastewater. recycled into proteins about 33% and 25% of the dissolved nitrogen missing from the medium 24 h after harvesting 50% and 70% of the culture, and replacing the volume harvested with fresh medium. The residual dissolved nitrogen concentrations were 25% and 43% of the initial, respectively, with an imbalance in the mass budget close to 17 and 20 mg N l(-1) dl. Most or all the nitrogen missing was found in an ammonia trap located at the air vent of the closed cultures, showing that an important role of microalgae in wastewater treatment is that of favouring NH3 stripping due to the photosynthesis-induced pH increases.