Retinal development of trisomy 19 (Ts19) mice has been studied by microscopy from postnatal day 2 (PD 2) to PD 15. Gross malformations of the eye or alterations of the cellular arrangement are not detectable; retinal differentiation follows the regular pattern observed in chromosomally balanced control littermates. At all stages examined, the development of the retina in Ts19 lags approximately 2 days behind littermate controls. The maturation of all cellular constituents examined is retarded to the same degree. Appearance of the outer plexiform layer, cessation of mitotic activity, penetration of blood vessels in the nerve fiber layer and in the outer plexiform layer, differentiation of the inner granular layer and receptor formation are each delayed about two days. On a cellular level, there is no indication for an asynchronous development in the retina of Ts19. In addition to the retarded maturation, the diameters of the eyes are reduced in Ts19 mice.