Abstract: Japanese foxtail (Alopecurus japonicus) is a malignant grass weed mainly infesting winter wheat- and canola-growing fields in China. To determine the resistance level and the potential resistance mechanism of A. japonicus to fenoxaprop-p-ethyl, 10 populations of A. japonicus were collected from the wheat fields in Tianchang City, Anhui Province. Whole-plant dose-response experiments were performed to investigate the resistance 1evels of different populations to fenoxaprop-p-ethyl in the greenhouse. Partial sequences of acetyl-CoA carboxylase (ACCase) genes were subsequently amplified, sequenced, and compared between the resistant and susceptible plants. The results revealed that all 10 populations displayed high-level resistance to fenoxaprop-p-ethyl, and the resistance indices ranged from 30.50 to 58.55. Gene sequencing showed that the mutations of isoleucine (Ile) to leucine (Leu) at 1 781 position and tryptophan (Trp) to cysteine (Cys) at 2027 position existed in the ACCase genes of different populations, with eight of them had mutations at 1 781 position. Besides, all populations had mutant frequencies higher than 80%. Our results indicated that herbicide resistance has evolved to a severe stage in Anhui Province, considering that all 10 populations collected from different sites showed high-level resistance to fenoxaprop-p-ethyl. The Ile-1 781-Leu and Trp-2 027-Cys mutations in ACCase genes were very likely to be the key reasons which caused the fenoxaprop-p-ethyl resistance of A. japonicus populations. In addition, compared with 2 027 position, ACCase seems more prone to produce a mutation at 1 781 position in A. japonicus.