In mice homozygous for the newly reported rds (retinal degeneration slow) gene, a progressive loss of visual cells occurs throughout life. Histogenesis of the retina proceeds normally with the exception of the receptor layer which remains rudimentary. Electron microscopic observations of the retina at different stages of development have revealed that the outer segments of the visual cells in this mutant fail to develop and that the receptor layer consists of inner segments only. In the absorption spectra of the retinal extract from the mutant the characteristic rhodopsin peak is lacking.