The hydrocarbon-rich green microalga, Botryococcus braunii, was grown on secondarily treated sewage (STS) in a continuous bioreactor system. The algal biomass increased at a sustained rate of 196 mg dry weight/l per week for 1 month. The hydrocarbon content of algae grown on STS (49%) compared well with that of algae grown on an artificial medium. The concentrations of nitrate and phosphate ions in STS decreased from 5.5 to 4.0 mg nitrogen/l and 0.08 to 0.03 mg phosphorus/l, respectively, by algal consumption. STS would thus appear to suffice as an appropriate medium for continuously sustainable growth of B. braunii and the algal consumption of nitrate and phosphate should help in removing these ions from STS.