The relative efficiency of various phosphorus fertilizers for cotton has been tested by the Instituto Agrônomico since many years. In this paper the authors report the results obtained in the first series consisting in six experiments started before 1941, five of which can be utilized to show the effects on the yields. The latter were conducted in the same plots for three or more seasons. The yearly applications of P2O5 varied according to the experiment from 60 to 90 kilograms per hectare and were always used in the presence of nitrogen and potash. The various phosphates tested, including ordinary and granulated superphosphate, precipitated phosphate, Rhenaniaphosphate, Thomas slag, and bone meal, gave practically the same results. The authors warn, however, that the small number of experiments and the conditions under which they were conducted do not as yet permit to draw definite conclusions on the subject. In four trials, which included plots fertilized only with nitrogen and potash, the mean effect of phosphorus on the yield of seed cotton varied from fair to very high. The latter offered an excellent opportunity for verifying the influence of phosphorus on some characteristics of the cotton plant and its fruits. Phosphorus increased also the weight and height of the plants, but not so much as the yield of seed cotton. When the first picking was made early, it was observed that phosphorus hastened the fruiting march. Phosphorus increased the weight of the bolls, but did not practically affect the lint percentage and length. The residual effect of phosphorus was determined in two of the experiments. Although they were conducted on the "terra-roxa-misturada" type of soil, that effect was high and no difference was observed between that of superphosphate and the relatively insoluble phosphates.