Weeds are one of the most important problems in onion (Allium cepa L.) production areas, since onion plants are poor competitors. This study was conducted in order to identify the weed species in onion fields in Cukurova Region, establish the effects of some herbicides on weeds and the yield of onion in reducing the existing phytotoxicity problem on onion plants between 2006 and 2007. A total of 105 weed species belonging to 30 families were identified in fifty onion fields that were surveyed. Top five species were found as Medicago polymorpha L., Convolvulus arvensis L., Avena sterilis L., Chenopodium album L. and Sinapis arvensis L. with 84, 74, 68, 66 and 66 frequencies (%), respectively. To determine the effects of herbicides on weeds and onion yield, the experiments were done in two different onion fields. Among the treatments, oxadiazon and oxyfluorfen provided a better control than pendimethalin and tepraloxydim on weed coverage and density, but oxyfluorfen caused phytotoxicicity on onion even at reduced doses. Weed-free plots gave the highest yield (16.2(a) kg/m(2)) and were followed by oxadiazon (11.9(b) kg/m(2)), oxyfluorfen (11.7(b) kg/m(2)), pendimethalin (10.0(c) kg/m(2)), tepraloxydim (9.6(c) kg/m(2)) and weedy check (9.1(c) kg/m(2)). Results, in which weed-free check caused 76.3% increase in the onion yields when compared with weedy checks, show that weed control is very important for onion production.