A Z-linked recessive mutation in the domestic fowl is reported that results in a keratoglobus of both eyes. It does not appear to have any effect on the vision of affected individuals. The keratoglobus is not noticeable at hatching, but becomes obvious by 5-6 weeks of age. The mutant has been designated pop-eye, gene symbol pop. Initial studies suggest linkage with the late feathering (K) locus, and with the NM 7092 t (Z;1) translocation, which had previously been shown to be closely linked with Z-linked silver (S). There appear to be no other detrimental effects associated with this mutant. Preliminary analysis of T3 and T4 thyroid hormone levels do not indicate any thyroid malfunction.