Four Chenopodium album L. (common lambsquarters) populations, two susceptible (S) and two putative resistant (R) were screened for resistance to four post-emergent acetolactate synthase (ALS)-inhibiting herbicides using whole plant spraying and enzyme inhibition assays. Thifensulfuron-methyl (sulfonylurea), imazethapyr (imidazolinone), thiencarbazone-methyl (triazolinone) and pyrithiobac-sodium (pyrimidinylthiobenzoate) were all applied at varying doses. Above ground biomass and visible injury data were taken nine days after treatment. Resistance to thifensulfuron-methyl was confirmed in the two putative resistant populations and cross resistance was discovered to the three other classes of ALS inhibitors. ALS inhibition assay revealed 157- to 729-fold-resistance to thifensulfuron and up to 6930-fold to pyrithiobac, confirming resistance due to enzyme target site modification.