Small eye (Sey) is a dominant mutation in the mouse affecting the embryonic development of the eyes and nose. In homozygous Sey/Sey embryos, the optic vesicles grow out but there is no lens induction and the nasal pits fail to develop. Scanning electron microscope studies of Sey/Sey embryos show that the maxillary processes develop normally and fuse with ridges of ectoderm in the frontonasal position. In Sey/+ heterozygotes, the vacuolated lens is smaller than normal, and there is folding of the margins of the optic cup and ingrowth of mesodermal cells. Evidence is presented that Sey is not allelic with Coloboma (Cm), another mutation affecting eye development on chromosome 2.