Abstract Aphakia, an autosomal recessive single gene mutation in the mouse, seriously affects the development of the ocular lens. Up to advanced stages of lens invagination morphogenesis proceeds normally. In the late lens cup and early lens vesicle stage, however, the epithelium of the lens rudiment becomes disorganized and the lumen of the vesicle fills up with rounded cells, apparently released from the epithelium. The lens stalk persists frequently. Probably as a consequence of the aphakic state other parts of the eye secondarily become abnormal. Immunofluorescent studies were done on embryonic normal and aphakia eyes with antisera against adult mouse crystallins. In the normal embryo the first positive reactions were found in the late lens cup stage ( 10 3 4 –11 days of gestation). By Day 12 all cells of the lens vesicle were brightly fluorescent. A day later the cells of the posterior wall, now lens fibers, had elongated sufficiently to obliterate the lumen of the vesicle. The entire organ was highly fluorescent, indicating that all of its cells contained large amounts of crystallins. The mutant lens, studied over the same time span, showed no reaction at all. The most likely explanation is, that the multiple structural genes, which normally must be involved in the production of the crystallins, are not expressed up to this time in the mutant. The combination of morphological and biochemical defects suggests that the gene involved in the mutation somehow functions in the control of lens differentiation.