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Atrazine is a herbicide applied to a number of important crops, but litle has been done to study the susceptibility of species like Digitaria horizontalis to such herbicide. This work was carried out to evaluate the effect of the stage of development of Digitaria horizontalis at spraying on the suppression imposed by atrazine. The experiment was conducted in 10 dm-3 pots, under greenhouse conditions, and the experimental units were disposed in a completely randomized design, in a factorial scheme 5x3, with four replicates. Treatments were composed by Five rates of atrazine (0; 0.5; 1.5; 2.5 and 4.0 kg ha-1), applied in three stages of D. horizontalis development (two leaves, four leaves and one-two tillers). Applications accomplished in later stages of development were inefficient in controlling this species, despite causing significant reductions of weed shoot and root dry mass and plant height. For applications at two leaves stage, a maximum of 60% control was achieved by using 4.0 kg ha-1 of atrazine, indicating the lack of efficiency of this herbicide in controlling this species.