A giant foxtail (PCW1) population putatively resistant to fluazifop-P and sethoxydim was identified in a carrot, onion, and corn cropping system in Wisconsin during 1991. Previous, extensive use of fluazifop and fluazifop-P over several years imposed a high level of selection intensity on grass weeds. In a field experiment, fluazifop-P, sethoxydim, and quizalofop at recommended dosages resulted in 58, 53, and 45% plant survival, respectively, within the PCW1 giant foxtail population; no plants survived treatment with clethodim or nicosulfuron, and few plants survived treatment with imazethapyr at recommended dosages. Based on shoot dry biomass reduction in greenhouse experiments, a PCW1 giant foxtail biotype had 16-, > 9-, 4.9-, and 4.2-fold resistance to fluazifop-P, diclofop, quizalofop, and fenoxaprop, respectively, relative to a giant foxtail (AC1) accession that was susceptible to aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicides. The PCW1 biotype had 134-fold resistance to sethoxydim and slight and inconsistent resistance to clethodim. The PCW1 biotype and AC1 accession were equally susceptible to imazethapyr, linuron, and oxyfluorfen. Based on plant survival, a PCW1 giant foxtail accession had 25- and > 143-fold resistance to fluazifop-P and sethoxydim, respectively, relative to the AC1 accession. The selection intensity associated with repeated use of fluazifop and fluazifop-P over 5 yr contributed to the cross-resistance of PCW1 giant foxtail to APP and CHD herbicides.