This study was designed to determine the effects of glyphosate (MON 14420) on earliness and lint yield of Roundup Ready^TM and conventional cotton varieties. The varietal evaluation was carried out in 1997 at "La Laguna" Experiment Station at Matamoros, Coahuila, Mexico. The experimental design was a split-plot with eight cotton varieties as the main plots and two herbicide treatments as the sub-plots. Glyphosate was topically applied in a 0.40-m band, when cotton plants were at four-leaf stage. Earliness was measured based on the date of first flower, date of first open boll and percentage of crop harvested at the first picking. Lint yield was also estimated. The date of first flower and the date of first open boll of transgenic varieties were not affected by the application of the herbicide glyphosate. The transgenic varieties PM 1244 RR and PM 1220 RR had the highest values of percentage of crop harvested. The varieties DP 5690 RR, PM 1244 and DP 5415 RR had the highest yields. Finally, The Roundup Ready gene gives to the transgenic varieties tolerance to glyphosate.