The effects of supplementary P (phosphorus) on the performance of cattle at two distinctly different experimental sites: one notorious for its P-deficient pastures (Armoedsvlakte), the other (Glen) suspected of being deficient in P, were compared over 5,5 years. The acuteness of the deficiency differed vastly between the two sites. At Glen no differences were observed in either bodymass or reproductive performance between supplemented (+ P) and unsupplemented (- P) cattle, whereas at Armoedsvlakte the cumulative mortality rate of – P cattle was twice that of + P (58,3 vs 27,3%). The – P group weighed on average 121 kg less and their reproductive performance was severely impaired. Calves of – P dams averaged 9,8% lighter at birth (34,0 vs 37,7 kg) and 21,6% lighter at weaning (181,8 vs 231,8 kg) than the + P group. Financial advantages of supplementation are discussed. Possibly the most serious effect of the deficiency is the depression of feed intake, especially during late lactation and early pregnancy: e.g. in 1979 the digestible organic matter intakes per day were 2752 and 4925 9 for – P and + P cattle, respectively. No such differences were observed at Glen. The + P cattle at both sites satisfied their ME and crude protein needs. A P deficiency uncomplicated by a protein deficiency was identified in the pasture of Armoedsvlakte.