Several weed-free and johnsongrass competition periods were established and maintained in cotton to evaluate their influence on cotton yield and reproduction of johnsongrass. When johnsongrass was sprayed with fluazifop in field plots 3, 6, 9, or 12 weeks after cotton emergence, only plots sprayed at 3 weeks yielded as much cotton as weed-free plots. Johnsongrass permitted to compete for 6, 9, 12, and 25 weeks reduced cotton yields 20, 60, 80, and 90%, respectively. When johnsongrass was transplanted into weed-free plots 3, 6, 9, and 12 weeks after cotton emergence, a weed-free period of 9 weeks was required to prevent significant cotton yield reduction. Cotton in plots weed free for 3 and 6 weeks yielded 81 and 89% as much seed cotton, respectively, as weed-free controls. Johnsongrass competing with cotton for 9 to 12 weeks produced 20 and 40% as many seeds and rhizomes, respectively, as weedy control plots. Some seeds and rhizomes were produced (1% of weedy check plots) in plots weed free for 9 to 12 weeks. Cotton grades were reduced in plots weed free for 3 and 6 weeks, and in plots where johnsongrass competed for 9 to 25 weeks.