The effect of weed competition on cotton (Gossypium hirsutum L.) was evaluated in the Centro Experimental de Campinas and the Estação Experimental de Tietê during the 1970/71, 1971/72 and 1972/73 agricultural seasons. In the first season, competition periods of 10, 20, 30, 40, 60, 90 or 120 days, after emergence were studied, compared to checks always either with or without weeds. In the next seasons, competition was studied during the first 10, 20, 30 or 40 days after emergence, or after these periods, until harvest, maintained previously weed free for these different periods and afterwards competition was allowed until harvest. The experimental plots had four rows of five meters, distributed in randomized blocks with five or six replications. For evaluation of the competition effects the following parameters were considered: number, fresh and dry weights of weeds, after each competition period; fiber percentage, boll and seed weight, cotton yield, fiber length uniformity, Micronaire, Pressley and maturity, at harvest. The results showed a lower yield of cotton when the competition period was longer than 20 days after emergence. When the crop was maintained weed free for the first 30 to 40 days yield was similar to the completely weed free treatment. Competition periods of 90 days or longer determined negative effect on yield. In some cases a negative effect of weed competition was observed on fiber percentage, boll and seed weight, Micronaire, fiber uniformity and Pressley.