A defect in the pupil shape of rosecomb bantam chickens was noticed by a breeder. The pupil in affected birds appeared to be elongated at the lower margin and consistent with a coloboma of the iris. Upon examination of all birds in the flock, the defect was found in 26% of females and 9% of males. Examination of breeders older than 1 yr led to the discovery of colobomas, although there were not enough birds to accurately determine the prevalence of the problem for previous generations. Affected birds were examined for other lesions, and none were found. Specific-pathogen-free (SPF) eggs were incubated and hatched with eggs from the affected flock as sentinels. No SPF chicks developed colobomas of the iris, while 37% of the bantams in the same hatch were affected. This experiment eliminated many possible infectious agents and incubator or brooder malfunction as causes of this eye lesion. An analysis of the flock pedigree suggested that the defect might be inherited and might be related either to feather color or to gender. No statistically significant correlation could be made between feather color and the iris colobomas. However, the trait was twice as likely to occur in females as in males, which suggests that it is a sex-influenced trait. Based on the determination that this was an inherited trait, a breeding strategy to reduce the prevalence of the lesion in which affected birds were removed from the breeding flock was developed and followed for 2 yr. The prevalence of colobomas was 22% in females and 2.9% in males after the first year and 7.8% and 0.8% in females and males, respectively, after the second year.