Abstract: Whole-plant bioassay experimernts were conducted to investigate the resistance level of Leptochloa chinensis (L.)Nees populations to cyhalofop-butyl in rice fields of eastern China. Results showed that population LC-17-276, LC-17-281, LC-17-282, LC-17-283, LC-17-289 and LC-17-290 had evolved high resistance to cyhalofop-butyl, with resistance index of 16, 21, 27, 30, 12 and 15, respectively. Molecular analyses revealed that the L. chinensis population LC-17-276 has a tryptophan (Trp)-1999-to-serine (Ser) substitution in the CT domain of the ACCase gene, whereas no mutation was detected for the other 9 resistant populations. To the best of our knowledge, this is the first report of Trp-1999-Ser mutation in L. chinensis. These results showed that L. chinensis was extensively resistant to cyhalofop-butyl in some paddy fields in eastern China.