Laboratory scale aerobic digestion experiments were performed and nutrient balances were maintained in order to evaluate nutrient transformations during digestion of waste activated sludge. Reactors loaded at 0.03 ft³ and 0.081 lb VS/day/ft³, were operated under both batch and daily feed conditions. Significant nitrification occurred, resulting in substantial reductions in pH and alkalinity. Denitrification occurred when dissolved oxygen concentrations dropped below 1.0 mg/l. Phosphorus and nitrogen were released to the liquid in approximately a stoichiometric ratio to the volatile solids destroyed. As much as 37% of the phosphorus and 41% of the nitrogen in the feed sludge was found in the supernatant after digestion. The solid phase of aerobically digested waste activated sludge had a nitrogen content of 7% of the suspended solids and a phosphorus content of 2% of the suspended solids.