The objective of this project was to characterise resistance to sulfonylurea herbicides in UK biotypes of the grass-weed Alopecurus myosuroides. Investigations included field studies, glasshouse bioassays, biochemical and molecular analyses. Responses to the selective sulfonylureas mesosulfuron-methyl + iodosulfiiron-methyl sodium mixture and flupyrsulfuron-methyl were investigated, while extensive use was made of the non-selective sulfonylurea herbicide sulfometuron-methyl as a screen for possible ALS target site resistance in A. myosuroides.