Two experiments to determine the effect of various rates of Ramrod herbicide on the development of onion seedlings were conducted on sandy-clay soil type, of Glacial formation, at the «Theodureto de Camargo» Experiment Station, Campinas. The results may be summarized as follows: 1 - Ramrod herbicide as pre-emergence application may be used for weed control on onion seedling formation. No apparent damage was noted with the application of the different rates of the herbicide. 2 - The comparison between Ramrod at 1.30 kg per hectare a. i. with the check treatment revealed no significant difference in the control of dicotyledoneous species that were found 22 days after sowing. 3 - The comparison of Ramrod at 1.95 and 2.60 kg per hectare a.i. of experiment 1, and 2.60, 3.90 and 5.20 kg per hectare a.i. of experiment 2 with the check treatment, showed highly significant differences in the control of dicotyledoneous species after 22 and 34 days. 4 - After 8 weeks no residual effect was observed from any rate of application of Ramrod. There was a regrowth of weeds after that period. 5 - Based on these experiments Ramrod at 1.95 up to 3.90 kg per hectare a.i., depending on soil type, could be recommended for weed control in onion seedbeds.