Cross sensitivity of sweet corn to multiple P450-metabolized herbicides is conditioned by alleles at or linked to the nsf1 locus on chromosome 5S. The sequence of the NSF1 gene is highly homologous with cytochrome P450 alleles. In 2005 and 2006, injury to 146 sweet corn hybrids differed substantially following post-emergence applications of three P450-metabolized herbicides (nicosulfuron, foramsulfuron and mesotrione) in 12 field trials in 6 states; leading to the hypothesis that the extent of injury was associated with the presence of non-functional P450 alleles. F₂ progeny of each hybrid were assayed in greenhouse and field trials for responses to nicosulfuron, foramsulfuron, and mesotrione. Ninety-two hybrids were classified as homozygous tolerant when none of 10,269 F₂ progeny were injured. Two hybrids were classified as homozygous sensitive when all 270 F₂ progeny were injured. Fifty-two hybrids were classified as heterozygous based on segregation of F₂ progeny that usually fit (Chi square, P=0.01) a 3:1 ratio of tolerant:sensitive plants. In the multi-state field trials, homozygous sensitive hybrids were killed by nicosulfuron and foramsulfuron and severely injured by mesotrione. Mean injury to homozygous tolerant and heterozygous hybrids 7 and 14 days after herbicide applications differed (t-tests, P=0.05) in 51 of 69 comparisons. Mean levels of injury to heterozygous and homozygous tolerant hybrids were plotted against trial means. Slope coefficients from regressions were 1.6, 1.3 and 1.4 (nicosulfuron, foramsulfuron and mesotrione, respectively) for heterozygous hybrids and 0.7, 0.88 and 0.7 for homozygous tolerant hybrids, indicating that average injury was about 60%, 30% and 40% greater than the trial average for heterozygous hybrids and about 30%, 12% and 30% less than the trial average for homozygous tolerant hybrids. These results support the hypothesis that the extent of injury to sweet corn due to these three P450-metabolized herbicides is associated with non-functional P450 alleles.