The effect of phosphorus limiting conditions on process performance was investigated using laboratory scale activated sludge units. Data were collected for COD:P ratios of 287:1, 212:1, 87:1 and 56:1 at several mean cell residence times. Data collected at COD:P ratios of 287:1 and 212:1 produced sludge with extremely poor settling characteristics and data were collected for only one mean cell residence time at each ratio; Data for COD:P ratios of 87:1 and 56:1 were used to estimate kinetic coefficients. These kinetic coefficients were used to develop predicted values of process performance subject to phosphorus limiting conditions utilizing mathematical and stoichiometric relationships. Predicted values were compared to laboratory data. For both predicted values and laboratory data, a phosphorus limitation resulted in decreased COD removal efficiency, nearly 100 percent phosphorus removal efficiency, decreased reactor solids concentrations, solids with poor settling characteristics, decreased sludge production and artificially high F/M ratios. / M.S.