During 2005 and 2006, fourteen and seven Phalaris minor populations were found in the wheat fields of Fars and Golestan Provinces, respectively, that exhibited resistance to fenoxaprop-P ethyl and diclofop methyl belonging to the aryloxyphenoxy propionate herbicides. Seed bioassay was conducted at the weed science laboratory of Ferdwosi University of Mashhad to study the resistance of the populations to the APP herbicides. Petri dish assay showed that the populations are resistant to applied herbicides with different level of resistance. Using the estimated parameters of concentration-response curves, discriminating concentration was determined fordiclofop methyl at 8.04 ppm and for fenoxaprop-P-ethyl at 1.05 ppm. A study was also conducted at the agricultural biochemistry laboratory of Cordoba University to investigate the biochemical basis of resistance to ACCase–inhibiting herbicides in the resistant populations. In vitro enzyme assays revealed a herbicide-resistant ACCase enzyme in the AR, MR4 and SR3 populations. Extracted ACCase enzyme from the shoots of these populations was highly resistant to both applied herbicides compared with the susceptible population. The results suggest that the mechanism of resistance to APP herbicides in the three most resistant populations (AR, MR4 and SR3) relates to an altered ACCase. In the case of the rest of the resistant populations, other mechanisms including enhanced metabolism, lack of absorption and translocation, and other unknown mechanisms may be involved. These results also confirmed seven populations are cross-resistant to both the herbicides studied.