The experiments herein reported deal mainly with the comparative effect of five sources of phosphate on the yield of the potato plant. The trials were carried out at three localities in the State of São Paulo, viz. : Campinas, Taubaté and Joanópolis. Soil analyses from the test plots showed variation in phosphorus content. This was low at Joanópolis, medium at Campinas, and high at Taubaté. Nitrogen was added as ammonium sulphate, nitrate of soda, and potassium nitrate ; potash, as potassium sulphate or potassium carbonate (ashes). Phosphorus was suplied by superphosphates (trade names of Dawco and Granulated, of American origin) by "Cibraphosphate", "Serranaphosphate" and "Serranaphosphatepotassico" from local manufacturers and by bone meal. All forms of nitrogen or potash tried were about equally effective for potatoes when applied with any of the phosphates. Phosphorus was found to be the most important element to increase production. Applications of only nitrogen plus potash, brought no significant increase in yield. The greatest yields were obtained with superphosphates in complete formulae. The other forms of phosphate fertilizers were better than the controls and the nitrogen plus potash treatments, but inferior to the superphosphates. The superiority of the superphosphates is ascribed to their greater solubility, which renders the phosphorus readily available to the potato plant.