Graduation date: 2007A suspected glyphosate-resistant Italian ryegrass (Lolium multiflorum) (OR) population was collected from a filbert (Corylus avellana L.) orchard near Portland, OR. Based on the dose-response experiments conducted in the greenhouse, it was found that the suspected glyphosate-resistant population was approximately fivefold more resistant to glyphosate than the susceptible (S) population. Furthermore, it was found that the S population accumulated between three and five times more shikimic acid than the OR population, confirming its resistance to glyphosate. The mechanisms conferring glyphosate resistance in two glyphosate-resistant L. multiflorum populations, one from Oregon, USA (OR), and one from Chile (SF), were also studied. Based on a Petri dish dose-response bioassay, the OR and the SF populations were two and fivefold more resistant to glyphosate when compared to the S population, respectively; however, based on a whole-plant dose-response bioassay, both OR and SF populations were fivefold more resistant to glyphosate than the S population. The S population accumulated two and three times more shikimic acid in leaf tissue 96 h after glyphosate application than the glyphosate-resistant OR and SF populations, respectively. There were no differences between the S and the glyphosate-resistant OR and SF populations in ¹⁴C-glyphosate leaf uptake; however, the patterns of ¹⁴C-glyphosate translocation were significantly different. In the OR population, a greater percentage of ¹⁴C-glyphosate absorbed by the plant moved distal to the treated section and accumulated in the tip of the treated leaf. In contrast, in the S and in the SF populations, a greater percentage of ¹⁴C-glyphosate moved to the untreated leaves and the stem. cDNA sequence analysis of the EPSP synthase gene indicated that the SF population has a proline 106 to serine amino acid substitution, which has been previously shown to confer moderate levels of glyphosate resistance. We confirmed glyphosate resistance in two different L. multiflorum populations, and found that they have different mechanisms of resistance. The OR population has limited glyphosate translocation while the SF population has a mutation of the EPSP synthase gene.