A novel cataract model was identified in the ddY strain (outbred colony) reared at Osaka Prefecture University. Opacity appeared as a white pinpoint focus in the unpigmented eyes of cataract mice at 6 weeks of age. All mice, fully viable and fertile, were bilaterally affected by the time they were 10 weeks of age. There were no gender differences in the incidence of cataracts. Histologically, 5-month-old cataract mice showed vacuolation of epithelial cells, disruption of lens fibers, and dislocation of the lens nucleus to the posterior lens cortex. To elucidate the mode of inheritance, heterozygous mutant hybrids between cataract mice and wild-type ddY mice, as well as offspring between the heterozygous mutants, were analyzed. No affected mice were observed among the heterozygous mutants, and the ratio of affected to unaffected mice was 1:3 among offspring between heterozygous mutants. For linkage analysis, we produced backcross progeny [cataract mouse x (cataract mouse x MSM/Ms mouse)], and concluded that the cataracts are inherited by an autosomal recessive gene. Moreover, the locus of the cataract gene, mct, was mapped to the 3.91 cM region encompassed by D2Mit467 and D2Mit320 on mouse chromosome 2 by linkage analysis. Thus, the present cataract mice represent a novel cataract mouse model, and have been designated Morioka cataract (MCT) mice.