Fertilizers treatments, either by foliar spraying or by soil applications were compared in the cotton crop using the IAC-12 variety with the aim of studying the effects of fertilizers on some physical characteristics of the fibre and the seed. The experiment was carried out on a latossol called «Terra Roxa Estruturada", at the Escola Superior de Agricultura "Luiz de Queiroz of the University of São Paulo, in the State of São Paulo, Brazil. Urea (46% Í), concentrated superphosphate (45% P2O5), potassium chloride (60% K2O) have been used as nitrogen, phosphorus and potassium sources, respectively. The results showed a positive effect on the fiber lenght, fiber strength, the skein strength of cotton yarns and higher seed weight (seed index), when potassium chloride has been sprayed on the leaves. In respect to the uniformitty of the fibre, and fiber fineness (fineness index), there was no influence of the foliar fertilization with none of the three macronutrients; best results have been obtained with the soil application of fertilizers. It must be pointed ou that with the higher rate of phosphorus (60 kg P2O5/ha) the fineness index value was igher. The rate of nitrogen (40 kg/ha) influenced the fiber-strength index, as determined by from Pressley index.