A semidominant mutation in the laboratory mouse, Coloboma (Cm), is described. Coloboma is located on Chromosome 2, as is the similar mutation Dickie's small eye (Dey). Coloboma has a moderately reduced expressivity. The anterior chamber is usually present in Cm/+. Both Cm and Dey show delayed detachment of the lens vesicle and microphthalmia, and homozygotes of both apparently die early in pregnancy.