The purpose of this study was to determine whether long-wavelength light (760 nm) promotes myopic shift in newborn guinea pigs during eye development. Newborn guinea pigs were raised in different types of illumination: long-wavelength light, mixed-wavelength light, and normal light. The guinea pigs that received long-wavelength light were raised in the illumination of long-wavelength light for 4 weeks and in the illumination of mixed light for another 2 weeks. The other animals were raised in the illumination of mixed light or normal light for 6 weeks. All the animals had measurements of refractive status and biometric parameters before and 2, 4, and 6 weeks after being raised in the illumination. Guinea pigs raised in long-wavelength light illumination developed a significant myopia, compared with those raised in mixed-light illumination after 4 weeks. Two weeks of recovery eliminated the refractive differences between the long-wavelength group and the mixed-light group. Compared with the guinea pigs raised in normal conditions, the guinea pigs raised in long-wavelength and mixed lights were more myopic and also showed longer vitreous chamber depth at 2, 4, and 6 weeks. These data suggest that illumination with monochromatic long-wavelength light may lead to myopia and to an abnormal visual experience in newborn pigmented guinea pigs.