During 2002 and 2003, two field experiments were conducted at the Agricultural Experiment Station of Lajas to evaluate phytotoxicity and efficacy of weed control of oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene] and halosulfuron-methyl [methyl 5- [[(4,6-dimethoxy-2- pyrimidinyl)amino]carbonylaminosulfonyl]-3-chloro-1-methyl-1-H-pyrazole-4- carboxylate] applied on three different dates and at three rates after onion (Allium cepaL.) planting. In the first experiment, early applications of oxyfluorfen at 0.13 and 0.27 kg ai/ha at 5,10 and 15 days after onion emergence (DAE) were less phytotoxic (≤ 30%) than simultaneous applications of halosulfuron-methyl, which resulted 100% phytotoxic at 0.027 and 0.054 kg ai/ha. Ten days after each application the lowest weed density (seven plants per 0.5 m2) was obtained with the oxyfluorfen treatment at 15 DAE; however, twenty days later the lowest weed density (five plants per 0.5 m2) resulted from the application at 5 DAE. Greater weed densities were obtained with both rates of halosulfuron-methyl (≥52 plants per 0.5 m2), regardless of application timing. In the second experiment, the applications of halosulfuronmethyl at 35 DAE to plots previously treated with oxyfluorfen at 5, 10 and 15 DAE, caused severe damage (≥80%) to onion. In relation to weed control, the best combination of both herbicides was the application of oxyfluorfen at 10 DAE followed by halosulfuron-methyl at 35 DAE; however, this treatment was not significantly better than the application of oxyfluorfen alone at 10 DAE. The use of oxyfluorfen alone at 10 DAE resulted in higher onion yield and bulb number than any of the other herbicide treatments. Halosulfuronmethyl application reduced onion yield by more than 97%; therefore, the use of this herbicide in this crop should be avoided. Key words: weed density, phytotoxicity, herbicides in this crop should be avoided.