The influence of the degree and mode of light limitation on growth characteristics of turbidostat cultures of Rhodobacter capsulatus was investigated using mass and energy balance regularities. Light limitation was achieved by increasing the steady-state biomass concentration at constant incident light intensity (similar to 100 W/m(2)) or by decreasing the incident light intensity at constant steady-state biomass concentration (similar to 500 mg of dry biomass/L). It was shown that under conditions of light limitation of Rh. capsulatus, the content of P and N in the biomass as well as the biomass degree of reduction were determined by the growth rate of the cultures. The energetic yield of biomass of Rh. capsulatus and total bacteriochlorophyll a content increased when light limitation increased. These parameters were higher in the cultures, in which light limitation was achieved by lowering the incident light intensity at low biomass concentration. This seems to be due to different distribution of lightwithin the photobioreactor when dissimilar modes of light limitation were used.