High sensitivity to near-ultraviolet light is a fundamental feature of vision in many invertebrates. Among vertebrates there are some amphibians, birds and fishes that are also sensitive to near-ultraviolet wavelengths. This sensitivity can be achieved through a class of cone photoreceptor containing an ultraviolet-sensitive pigment. Although these receptors were thought not to exist in the eyes of mammals, we now report that some rodents have a retinal mechanism that is maximally sensitive to ultraviolet light.