Five green foxtail populations and one giant foxtail population were suspected of resistance to acetohydroxyacid synthase (AHAS) inhibitors. This study aimed at confirming resistance and determining its physiological and molecular basis. The response to four AHAS inhibitors was determined at the whole plant and enzyme levels. 'AHAS' genes were sequenced and Southern hybridization analysis was used to document 'AHAS' gene copy number. Resistance was confirmed at the whole plant and enzyme levels in all suspected populations. All green foxtail populations had resistance to imazethapyr with cross-resistance to nicosulfuron and flucarbazone but only three populations had cross-resistance to pyrithiobac. Giant foxtail was resistant to all inhibitors. Resistance in green foxtail was caused by single base-pair mutations in the 'AHAS' gene coding for four amino acid substitutions (Ser653Thr, Ser653Asn, Ser 653Ile and Gly654Asp). Two 'AHAS' genes were confirmed in giant foxtail; a single base-pair mutation coding for Ser 653Asn was found in gene II.