Seed cotton (Gossypium hirsutum L.) yields were not reduced when competing with prickly sida (Sida spinosa L.) for 7 weeks or less after cotton emergence and maintained weed-free for the remainder of the season. Cotton yields were not affected when pure prickly sida stands were controlled for 5 to 6 weeks after cotton emergence and then allowed to grow uncontrolled for the remainder of the season. In the more severe weed competition treatments, cotton plant height and main stem diameter were reduced; however, yield of seed cotton was the most sensitive indicator of prickly sida competition. Maturity of cotton, as measured by percent of total yield obtained at first picking, indicated a delay in maturity in 1 of 3 yr in treatments where competing prickly sida was present for 8 weeks or more. In density studies, where prickly sida emerged with cotton and was allowed to compete for the full growing season, 45 prickly sida plants/15 m of row caused reductions in yield of seed cotton in two of five experiments. In another series of three experiments as few as 32 prickly sida plants/15 m of row significantly reduced seed cotton yields in two of the three experiments. Trash content of machine-harvested cotton was increased at densities of 64 and 128 prickly sida plants/15 m of row in one experiment. Staple length and grade of cotton were not affected by any weed density studied. Micronaire measurements of cotton were reduced at prickly sida densities of 64 and 128 weeds/15 m of row during 1972.