Microphthalmia is a new mutation of the campbelli hamster (Phodopus campbelli) that is controlled by an incomplete dominant autosomal gene Mi. The dorsal coat of the heterozygote had dark markings on a white background. The dark markings appear on the head, back, and rump. Their color is very similar to that of the wild type, but slightly lighter. The pupil and iris of the heterozygote are black with reddish tinge. The homozygote has pure white fur all over the body and shows a smaller body size than the wild type and heterozygote. Moreover, the homozygote is characterized by small eyes with unopened eyelids and loss of the incisors. The pupil and iris are colorless and transparent or show a faint reddish tinge when the eyelid is artificially opened. In addition to these abnormalities, individual bones of the homozygote show dwarfism. The parietal and frontal region of the skull are thin and their symphysis is incomplete. The distal regions of the vertebral ribs show swelling. The homozygote is basically lethal within 3 weeks of age. A very few survivors are sterile.