Induction of genes conferring herbicide resistance by mutagenesis could facilitate use of imidazolinone herbicides in upland cotton (Gossypium hirsutum L.). In 1997 and 1998, seeds of eight High Plains cotton cultivars were treated with 2.45% v/v ethyl methanesulfonate. The resulting M3 and M4 generations were sprayed with imazethapyr, and the M5 and M6 generations were sprayed with imazamox. Four stable M6 to M7 lines with tolerance to imazamox were identified in 2004. During 2005 and 2006, tolerant mutants and their nonmutated parents were treated at the four-leaf stage with topical applications of imazamox applied at five rates (0, 88, 175, 350, and 700 g a.i. ha-1). Elevated levels of tolerance to imazamox were observed in all mutants. Imazamox did not impact fiber length. Preliminary investigation indicated that tolerance to imazamox was controlled by a partially dominant single gene. An allelism test revealed that the tolerance genes in the four mutants are either alleles in the same locus or are very tightly linked.