The effect of boron application on cotton at planting time was studied in a deficient red-yellow latosol sandy phase soil, located at Leme, State of Sã Paulo, Brazil. Field experiments were conducted during three years namely 1976/77, 1977/78 and l978/79. In the first year, boron was applied at the levels of 0, 0.5, 1.0, 1.5 and 2.0kg/ha of B, as borax (11% of B). In the subsequent years, levels of 0.75 and 1.25kg/ha of B were introduced in the experimental area. A latin square design was used in all the trials. A positive effect on cotton yield resulted from boron fertilization. The response to boron was greater at a higher level of plant productivity. With regards to economical aspects, the optimum rate of boron was 1.23kg/ha of B. The influence of micronutrient on earliness and weight of bolls was statistically significant and of a quadratic form, also. Otherwise, boron decreased fiber maturity. Plants of check plots showed severe boron deficiency symptoms such as abnormalities of flowers, dark rings and pith necrosis in leaf petioles and in stems, and plant with longer vegetative cycle, excessively branched, with enlarged, dark and ruptured nodes.