Abstract: The purpose of the project was to determine the value of using lagoons as a supplemental process for treating the effluent from an activated sludge wastewater treatment plant. Only tentative conclusions can be suggested since the project was not carried to completion. It appears that lagoons will have only minimal effect on the amounts of total nitrogen and phosphate in the plant effluent, although conversion of a portion of the ammonia to nitrate can be expected. However, the concentration of ammonia was never lower than that required by EPA guidelines. Some reduction of soluble BOD appears to occur, perhaps through conversion to new cell material. Suspended solids concentrations can be expected to increase during periods of algae growth. The numbers of both fecal and total coliform bacteria decreased substantially during the detention period provided by the lagoon system. Although not confirmed, it was concluded that the most probable cause for the decrease was the intensity and duration of sunlight. The lagoon served very effectively as a buffer between the treatment plant and the receiving stream, providing a considerable measure of protection to the stream even during those periods during which the plant was by-passing a portion of the inflow. Funding information: The work on which this report is based was supported in part by funds provided by the Office of Water Research and Technology, U.S. Department of the Interior, as authorized under the Water Resources Research Act of 1964.