During 2002-2003 growing season, two field experiments in Fars and Khuzestan provinces and three greenhouse experiments at Plant Pest and Disease Research Institute, Tehran, were performed to evaluate the resistance of wild oat to aryloxyphenoxy propionate herbicides. Seed populations of wild oat were collected from nine locations in Fars and Khuzestan, in which seven populations were expected to be resistant to fenoxaprop-p-ethyl or clodinafoppropargyl and two of them were considered susceptible to these herbicides. In field experiments, fenoxaprop-p-ethyl at 75 g ai ha-1 was used in Fars and clodinafop-propargyl at 64 g ai ha-1 in Khuzestan to examine the response of wild oat populations treated at three-leaf stage to these herbicides. In greenhouse experiments, the response of 12 wild oat populations collected from three provinces (Fars, Khuzestan and Markazi) to fenoxaprop-p-ethyl, clodinafop-propargyl, and diclofop-methyl at 900 g ai ha-1 were examined in separate experiments. In each experiment, an untreated control of wild oat plants was maintained for each population (except Fars populations). Wild oat density, shoot dry weight, and visual rating of weed control were recorded. The results showed that wild oat populations in Khuzestan have developed resistance to aryloxyphenoxy propionate herbicides. But wild oat populations of Fars and Markazi are still controllable by this group of herbicides. However, there were some differences between the results of field experiments and those of the greenhouse experiments. For example, one of the populations (KR3) obtained from Khuzestan which was resistant to clodinafop-propargyl in field experiments only, showed resistance to all grass herbicides used in greenhouse experiments. In addition, two (KRI and KR2) of them were resistant to all herbicides in greenhouse experiments only. Among populations collected from Fars, only one FR4 of them was resistant only to fenoxaprop-p-ethyl in greenhouse experiments. This indicated that the level of resistance of weeds is probably influenced by the environmental conditions.