Cotton meal was studied as a source of nitrogen for Irish potatoes in six trials carried out in five localities of the State of São Paulo. Six fertilizer mixtures, containing equal amounts of superphosphate and potassium sulphate, and varying proportions of cotton meal and ammonium sulphate, were compared. The results of these tests indicated that the two sources of nitrogen increased the potato yield ; and that when the amount of soil moisture is satisfactory, nitrogen may be supplied by cotton meal alone or in mixture with ammonium sulphate. In one trial the cotton meal had no effect on yield, whereas the ammonium sulphate was effective. Also no injury to the plants resulted from application of cotton meal. Based on the results of these trials it was concluded that cotton meal can be used to advantage in fertilizers for Irish potatoes, preferably in mixture with ammonium sulphate, at the rate of 650 kg of cotton meal per hectare.