1. The yield of cotton strains derived from Delfos 719 and from Stoneville 2B was compared in trials with the commercial variety I. A. 7387 (control) at the Ribeirão Preto Agricultural Experiment Station for five consecutive years (1937/38 - 1941 /42). 2. Annual yield varied considerably during these years. Low yields dominated. 3. Number of bolls formed was found to be correlated to quantity of rain and its distribution during the flowering time. Too much rain weighs against yield lowering it, probably on account of the increase of shedding. 4. In 1937/38 a bulk of several strains of Delfos surpassed statistically the control in 18%. 5. In the four subsequent years, strains of Delfos and Stoneville 2B, isolated in 1937/38, proved to be far superior to I. A. 7387. 6. All strains of Stoneville 2B and eight strains of Delfos showed less variation in production than I. A. 7387, showing that they are less influenced by the environmental factors. 7. The best strain of Delfos, the one 38/1418 surpassed I. A. 7387 in yield during four trial years, averaging 1,348 pounds of seed cotton per acre against 1,046 pounds, or 28,9% above the control. 8. Among strains of Stoneville 2B, the one 38/1709 proved to be the best; its yield was superior to that of the control in three out of four years. During four years the average yield of this strain was 1,262 pounds of seed cotton per acre or 20,6% above the control. 9. Delfos strain 38/1418 and Stoneville 38/1709 could be used in the improvement of cotton production in the district of Ribeirão Preto.