The molecules essential to the continual morphogenesis and shedding of the opsin-containing disks of vertebrate photoreceptors are largely unknown. We describe a 37 kd protein, rom-1, which is 35% identical and structurally similar to peripherin/retinal degeneration slow (rds). Like peripherin, rom-1 is a retina-specific integral membrane protein localized to the photoreceptor disk rim. The two proteins are similarly oriented in the membrane, and each has a highly conserved (15/16 residues) cysteine- and proline-rich domain in the disk lumen. Although both rom-1 and peripherin form disulfide-linked dimers, they do not form heterodimers with each other, but appear to associate noncovalently. These results suggest both that rom-1 and peripherin are functionally related members of a new photoreceptor-specific protein family and that rom-1, like peripherin, is likely to be important to outer segment morphogenesis. The association of mutations in RDS with retinitis pigmentosa indicates that ROM1 is a strong candidate gene for human retinopathies.