The ocular retardation (or) mutation in mice has been studied morphologically in serial 1 mu sections. This recessively inherited, fully penetrant mutation is characterized by an early arrest of retinal development, aplasia of the optic nerve, cataractous degeneration of the lens, and microphthalmia. We describe early alterations of normally occurring morphogenetic cell death in the optic cup and aberrations of optic fissure formation which appear to precede the arrest of retinal and optic nerve development. The subsequent disappearance of central retinal vessels and cataract formation are interpreted as secondary phenomena.