Three experiments were conducted in 1975 at Fortaleza and Pentecoste, State of Ceará, to de termine the effectiveness of glyphosate in the control of purple nuts edge (Cyperusro tundus L.) in soil without a growing crop. The treatments were as follows: Control (without glyphosate); and 6 levels of as 1, 2, 3, 4, 5 and 6 kg of active ingredient per hectare. The material was applied as a post - emergent when the nuts edge was prior, or in the flowering stage. Only one aplication was made for each of the treatments. Under the conditionns of thes e experiments, the following conclusions can be drawn: 1. Glyphosate, at levels of 2 and 3 kg(ia)/ha, gave a satisfactory degree of control for 30 days with only one application; 2. Glyphos at e must be applied when the nuts edge is in the 6- or 7 - leaf stage and in rapid vegetative development and before the initiation of flowering; and, 3. Higher level sof application are not recommended because the degree of control is similar to that obtained at 3 kg(ia)/ha.