EXCESSIVE enrichment, or eutrophication, of receiving waters by nutrient-rich wastes is emerging as a major water pollution problem. For some time it has been recognized that ordinary domestic sewage is a rich source of the nutrients required by phytoplankton. Inorganic nitrogen and phosphorus are the most important of these nutrients, and it is on the supply of these that the degree of eutrophication depends. From time to time, interest has developed in finding some method of waste treatment which would remove the offending fertilizing elements from sewage before it is discharged into the receiving water. Limnological investigations indicate that removal of phosphorus offers a practical and effective way of controlling algal growths in most natural waters (1). Phosphorus may be removed by biological or chemica,l means. Both approaches aim at the convetsion of soluble inorganic phosphorus into recoverable insoluble matter. Of the two, the chemical method has received greater attention, and several promising but costly means of chemical coagulation have been proposed. Little has been done with biological treatment. The usefulness of algae as a means of remnoving nutrients from domestic sewage is examined in this paper. This aspect of algal activity appears to have received relatively little attention; hence, much of what follows is based on research carried out at the University of Washington from 1956 to 1959. Wherever possible, pertinent data reported by others have been drawn upon.