A formulated mixture of two sulfonylurea herbicides, mesosulfuron and iodosulfuron, combined with the safener mefenpyr-diethyl ('Atlantis') is being used extensively in Europe and there is concern that resistance will evolve in Alopecurus myosuroides (black-grass). Glasshouse screening bioassays showed that the best single discriminating dose for detecting resistance is the UK field rate of 12 g mesosulfuron + 2.4 g iodosulfuron ha(-1) applied at the 3 leaf stage, with herbicidal effects recorded 4 weeks later. Using this methodology with 466 UK seed samples, resistance was confirmed on a total of 24 farms in 11 counties by 2005, 81 farms in 19 counties by 2006 and 133 farms in 21 counties by 2007. Cultural histories for 10 resistant (R) and 7 susceptible (S) fields were obtained. Winter cereals were grown in 73% R/ 71% S years and a mean of 3.0 grass-weed active ingredients applied per year in both R and S fields. Four herbicide classes dominated, comprising almost 80% of all applications: ALS inhibitors 17% R/ 21% S; ACCase inhibitors 19% R/ 17% S; substituted ureas 18% R/ 10% S; dinitroanilines 25% R/ 30% S. Consequently, ALS inhibitor use was not excessive and field histories were not a reliable indicator of resistance risk. DNA sequencing of the ALS gene from resistant and susceptible individuals of nine populations was used to identify resistance mechanisms. All highly resistant individuals from seven populations showed a single nucleotide polymorphism in the first position of the Pro197 codon of an A. myosuroides ALS gene. One population showed resistant individuals with a single nucleotide polymorphism in the second position of the Trp574 codon. Consequently ALS target site resistance was confirmed in eight of the nine populations studied in detail.