This paper reports the results obtained in two pot and eight field experiments with potatoes, which were conducted from 1943 to 1947 and designed to study the effect of increasing rotes of superphosphate (30, 60, 90, 120, 180 and 240 kilograms of P2O5 per hectare) in the presence of nitrogen ond potash (ammonium sulphate and sulphate of potassium). The pot tests were conducted in the Central Experiment Station, Campinas, and the field trials were carried out on different areas of four localities of the State of São Paulo. The responses to phosphorus were generally very good and the average of all the rates of the field experiments reached +42%. In the average the yields increosed rapidly when the doses of P2O5 were raised up to 120 kilograms per hectare, continued increasing, but more and more slowly, till the 180 kilograms dose, and kept practically unchanged with higher doses. Distinguishing the experiments localized on soils fertilized with phosphorus in the previous crops from those conducted on soils which were never fertilized, in the former group the tendency was far the yields to keep unchanged with doses higher than 120 kilograms, while in the latter the yields increased even when the highest experimental rate was used. According to the current prices and the general average of the eight field experiments, the most profitable rates af P2Or, application would vary from 120 to 150 kilograms per hectare; for the soils previously fertilized with phosphorus the applications should be of about 100-120 kilograms whereas over 150 kilograms could be used profitably in the never fertilized soils. The phosphorus fertilization increased the size of the tubers and their phosphorus content, but did not modify the density, the capacity of conservation and the starch content of them, nor did it influence the incidence of internal brown spots.