The development of the root system of two cotton varieties grown in the three main soil types of the state of S. Paulo was studied. Under the conditions tested, cotton roots reached very deep, in some cases as deep as 2.5 m. The variety I. A. 7387-24940 had a deeper and denser root system than the I. A.Campinas 817. It was found that the root system of cotton plants grown in the "terra-roxa-misturada" soil was the deepest, followed by the root system of plants grown in "massapê" and then in "arenito de Bauru". Regardless of the variety or soil type, 80% of the total root system occurred to a depth of 20 em. Due to this fact emphasis is given to shallow cultivation in cotton plantings as a mean to prevent damage to the root system of the plants.