Examination of mouse strains with a slit lamp and indirect ophthalmoscopy revealed that strain CBA/CaGnLe has a white cataract obvious at weaning age. It soon progresses to a large white nuclear cataract with mild cortical changes. Crosses with C57BL/6J showed that this is inherited as a single recessive fully penetrant gene, which we have designated lop18 (lens opacity 18). Linkage analysis using visible marker T (brachyury), histocompatibility marker H2, and microsatellite markers D17Mit21, D17Mit28, D17Mit38, and D17Mit46 shows that the lop18 gene is located, approximately 16 cM from the centromere on mouse Chromosome 17. It is a likely candidate mutation for the alpha-crystallin (Crya1) gene.