Trisomy 1 embryos consistently show eye defects (e.g., aphakia, microphakia, retention of lens stalk). To determine if the plane of division of mitotic figures is abnormal in the eyes of these animals, trisomic embryos (9.5 through 12 gestational days) were produced from mice doubly heterozygous for Robertsonian translocation chromosomes [Rb(1.3)/Rb(1.10)]. To accommodate for the known delay in trisomic embryo development, animals were grouped according to stages of eye development rather than to gestational age. Serial sections were evaluated without knowledge of karyotype for orientation of mitotic figures (parallel, perpendicular, oblique) in lens, optic cup, and diencephalon. Location of mitotic figures was scored as apical (nearest the lumen), middle, and basal. Numerous anomalies were noted in trisomic eye development. No difference was found between orientation of mitotic figures in the lens and optic cup of trisomy 1 and normal embryos. Location of mitotic figures in trisomy 1 lens was significantly different from that of normal littermates. The data confirm earlier studies that trisomy 1 affects the eye, and they tend to corroborate evidence that this trisomy affects the lens more than it affects the optic cup.