Xanthommatin is the primary ommochrome eye pigment in mosquitoes. The terminal step in xanthommatin biosynthesis, involving oxidation of 3-hydroxykynurenine (3HK), can proceed enzymatically by phenoxazinone synthase or by nonenzymatic auto-oxidation of 3HK. The relative contributions of these pathways, however, are unclear. We isolated a novel Culex pipiens mutant (crimson) that could be used to address this question. Homozygous crimson embryos exhibit no visible eyespot; first-instar larval ocelli are colorless. Eyes gradually turn red through immature development. Teneral crimson adults possess red eyes that darken to wild-type ≈5 d after emergence. Crosses indicate that crimson is sex-linked and fully recessive. Addition of xanthommatin precursors to rearing water did not rescue wild-type phenotype and suggested that the mutation is in the terminal step of ommochrome biosynthesis. Crimson expression was not temperature sensitive. Thin-layer chromatography demonstrated teneral crimson adults lacked xanthommatin. Teneral and aged wild-type adults exhibited low-mobility black ommochrome spots; aged crimson adults exhibited low-mobility brown-red ommochrome spots. Absorbance spectroscopy of eye extracts indicated teneral adult crimson eyes lacked xanthommatin but had abnormally high levels of 3HK, whereas extracts of 10-d-old crimson adults had depleted levels of 3HK and detectable levels of xanthommatin. Light microscopy indicated that eyes of young (3 d old) wild-type adults had a high concentration of pigment granules. Eyes of teneral crimson adults had no pigment granules. Eyes of 20-d-old crimson adults had low levels of pigment granules. We suggest two possible mechanisms for the crimson mutation: (1) transport of 3HK into the pigment cells and/or pigment granules is slow, with normal oxidation of 3HK into xanthommatin, or (2) 3HK is transported normally into pigment cells/granules but is not immediately oxidized to xanthommatin, resulting in 3HK hyper-accumulation and slow nonenzymatic production of xanthommatin after adult emergence.