The Emory mouse cataract is a late-appearing lens opacity which may serve as an animal model for some human senile cataracts. It is inherited as an autosomal dominant trait and has a typical course of development. Lens opacities may become readily apparent as early as 6-8 months in mice having a familial history of early cataracto-genesis. Many gross morphologic and microscopic features resemble findings in human senile cataract. As an animal model it has many desirable characteristics. Its slow development permits studies of the lens at the pre-cataractous stage and makes it a good assay system for drugs or other factors affecting cataractogenesis. In this paper are given some morphologic and histologic aspects of the developing cataract.