The tolerance to 2,4-dichlorophenoxy acetic acid (2,4-D) of a genetically modified (transgenic) cotton (Gossypium hirsutum) expressing a 2,4-D detoxification gene was compared with conventional (non-transgenic) cotton over 2 seasons. The 2,4-D was applied over-the-top of cotton in the field at 7–17 nodes of crop growth at rates of 0.004–1.12 kg a.i./ha. The transgenic cotton displayed better tolerance to 2,4-D than conventional cotton at all growth stages and herbicide rates. Some damage was apparent on both types of cotton at 2,4-D rates of 0.07 kg/ha and above, with damage most pronounced when the plants were exposed at 7 nodes. The transgenic cotton also had some tolerance to MCPA. Commercial use of transgenic, 2,4-D-tolerant cotton has the potential to greatly reduce problems of 2,4-D damage in cotton from accidental spray drift and herbicide residues in spraying equipment, where plants are predominantly exposed to low rates of 2,4-D.