In this paper the authors study four groups of experiments, comprising a total of 32 annual results, in which the time of thinning cotton varied from 15 up to 80 days after the emergence of the plants. The experiments were conducted between 1927-28 and 1943-44 and located at several places of the State of São Paulo. On the average, as thinning was delayed the stands became a little better, but the yields of seed cotton decreased considerably; while the lint percentage and length diminished slightly, the bolls' size and the contribution of the first picking to the final yields suffered appreciable reductions. The authors conclude that cotton should be thinned 20 to 30 days after the emergence of the plants.