Newborn hamsters were inoculated intracerebrally with either the neurovirulent Kilham strain of mumps virus or a mutant (M13) strain of this virus. The M13 strain has an alteration in the haemagglutinin-neuraminidase protein of its envelope and causes a low-grade infection of the brain. Both strains spread consistently to the retina where the Kilham strain caused an extensive necrotizing infection. In contrast, the M13 strain predominantly caused an infection of the retinal pigment epithelium (RPE) with the involvement of scattered neurons in the retina. Only minimal degenerative or inflammatory changes were seen, but at 12 days of age developmental alterations were seen in all eyes. These included stretches with failure of photoreceptor segment development and the formation of folds in the outer nuclear layer. The former changes occurred in areas with loss of RPE cells and the latter generally in connection with displaced pigment-loaded cells from the RPE layer. It is suggested that these retinal alterations are mainly secondary to the RPE infection with the M13 strain.