An alkaline phosphatase assay based on the use of a p-nitrophenyl phosphate substrate was studied for its applicability to indicate surplus phosphorus uptake by a dilute heterogeneous bacterial culture obtained from an activated sludge process. This investigation demonstrated the validity of the enzymatic bioassay using viable bacterial cells and determined conditions under which the bioassay should be conducted as a standard procedure. Alkaline phosphatase activity was found to be repressed by both orthophosphate external to the cells and by stored phosphorus within the bacterial cells. Based on these studies, the assay appears useful to help elucidate the mechanism responsible for the abnormally high phosphorus removal being attained by various activated sludge plants in the United States. The research also indicates the possible significance of the surplus phosphorus uptake phenomenon in waste treatment processes. (Author)