The "attached cultivation" method of microalgae in which the wet paste of algal biomass is attached onto supporting materials to form an immobilized biofilm layer, and the culture medium is supplied to this layer to provide nutrients and moisture for growth was highly efficient in biomass production and represents a promising technology to improve the biofuel industry. To optimize the nitrogen supply strategy for this attached cultivation method, the growth and total lipids accumulation properties for the green alga Aucutodesmus obliquus with this method were studied under different quantities of nitrogen source and different volumes of aqueous medium that continuously circulated inside the photobioreactor. Results showed that, compared with medium volume, the nitrogen quantity was a stronger factor affecting the growth and total lipid accumulation. An optimized nitrogen supply strategy for the attached cultivation of A. obliquus is proposed as circulating ca. 60 L of BG-11 medium containing 1/10 of nitrate concentration for 1 m(2) of cultivation surface. With this strategy, the attached A. obliquus accumulated biomass and total lipids simultaneously and obtained a high triacylglyceride productivity of 2.53 g m(-2) day(-1) in 7 days under subsaturated illumination of 100 mu mol photons m(-2) s(-1). The water usage of 60 L m(-2) was potentially decreased to < 2 L m(-2) if the nutrient supply was further improved. Dissolving the nitrogen source in small volume was the best way to efficiently utilize the nitrogen source with minimum of waste.