Laboratory experiments were conducted to determine the need for addition of phosphorus during the treatment of pulp and paper mill waste waters by the activated sludge method. The study also included the testing of different modifications of the activated sludge method (a completely mixed, three completely mixed reactors in series, anaerobic/aerobic) to see how different forms of phosphorus and nitrogen (total-N, NH+4 - N, NO-2 - N, NO-3 - N) are present in the influent and effluent. The tests were conducted using waste water from two newsprint/magazine paper mills and from a bleached sulphate pulp mill. Different loadings and levels of phosphorus addition were applied. When paper mill waste water was treated at normal loading (sludge load was c. 0.3 kgBOD/(kgMLVSS*d)), a small phosphorus addition was needed to secure efficient operation. The optimum BOD:P ratio was about 100:0.4, in which case the treated effluent had a total phosphorus content of c. 0.5 mg/l (about 70% reduction), a soluble phosphorus content of c. 0.3 mg/l and a phosphate phosphorus content of well below 0.1 mg/l. Larger phosphorus additions produced no further improvement in treatment results (BOD reduction c. 90% and COD c. 75%). Doubling the loading gave poorer results and the situation could not be rectified by adding phosphorus. Addition of phosphorus was not needed when treating pulp mill waste water, as has also been found when running activated sludge treatment plants at several mills. The BOD reduction (c. 95%) was excellent under all conditions. The COD reduction was 30-55%, AOX 30-35% and chlorophenols 90-95%. The total phosphorus content of the treated effluent was 0.3-0.7 mg/l when no phosphorus was added. This treatment also resulted in extremely low phosphate phosphorus levels. The biosludge contained 0.5-1.9% phosphorus, 0.5-0.8% when pulp mill waste waters were treated and occasionally around 2% for the paper mill. The experiments showed that it might be possible to operate the pulp mill treatment plant with even less phosphorus in relation to BOD compared with the BOD level of waste waters to which no phosphorus has been added. The mill could consider removing the excess phosphorus originating from lime mud neutralization before the waste water arrives at the treatment plant. In treating both these waste waters there is the risk of really high phosphorus discharges if care is not taken with the phosphorus addition. A typical situation of this type arises if the plant is run on the old "textbook rule" of BOD:P=100:1. The nitrogen was added as urea resulting in the BOD:N ratio of 100:(2.5-4.5). Total-N in the paper mill untreated waste water was in the range of 8.5-13 mg/l and in the effluent 2.5 - 5.0 mg/l, i.e. the removal was 55-75%. NH+4 - N in the influent was in the range of 1.5-3.0 mg/l and was totally removed in most of the runs. The concentration of (NO-2 - N + NO-3 - N) was only 40-50 µg/l, the removal was 0-85 % depending on the conditions. The activated sludge modification "three completely mixed reactors in series" yielded the best results when all parameters were taken into account.