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Italian ryegrass (<i>Lolium multiflorum</i> L.), a cross-pollinated grass, is gradually becoming a predominant weed in wheat fields in China and is evolving resistance to many groups of herbicides. The aim of this study is to determine the resistance levels of a single <i>L. multiflorum</i> population from a wheat field in Henan Province China, to three modes of action (MoAs) of herbicides and to further characterize the potential resistance mechanisms. This <i>L. multiflorum</i> population evolved multiple herbicide resistances to pyroxsulam [acetolactate synthase (ALS)], pinoxaden [acetyl-CoA carboxylase (ACCase)] and isoproturon [photosystem II (PSII)]. Target-site resistance (TSR) mutations (Pro-197-Gln, Pro-197-Thr, and Trp-574-Leu) and non-target-site resistance (NTSR) mediated by cytochrome P450 monooxygenase (CYP450) genes were associated with pyroxsulam resistance. Pinoxaden resistance was conferred by two TSR mutations, which referred to a rare Ile-2041-Val mutation and a common Ile-1781-Leu mutation but with two different nucleotide substitutions (CTA/TTA). CYP450- and glutathione-S-transferase (GST)-mediated resistances were the main resistance mechanisms for this multiple herbicide-resistant (MHR) population to the PSII inhibitor isoproturon. This is the first case of a single <i>L. multiflorum</i> population evolving multiple resistance to three herbicide MoAs (ALS, ACCase and PSII) in China. Diverse resistance mechanisms including TSR and NTSR mean <i>L. multiflorum</i> exhibits a high degree of resistance plasticity.