The California ground squirrel (Spermophilus beecheyi) has two classes of cone photopigments (lambda max = 440 and 525 nm). Under photopic conditions about 30% of all optic nerve fibers receives inputs from both cone classes, the remainder are driven solely by the 525 nm cone. Recordings were made from optic nerves in young ground squirrels to trace the development of their spectral responsiveness. Animals were reared from birth in one of three photic environments: white light, darkness, or red light. In the youngest ground squirrels examined (ca. 50 days old) many units receiving inputs from the 525 nm cone were found, but the proportion of units receiving inputs from both cone classes was significantly lower than that of adults. From these initial low levels the proportion of such units increases gradually. Adult proportions were achieved at varying rates which depended on the photic environment in which the animal was reared: animals reared in white light achieved the adult standard earliest, those reared in darkness somewhat later, and the animals reared in red light required much longer to achieve the adult organization. We conclude that: (a) the neural substrates for normal color vision in this species develop to some extent postnatally, and (b) the normal sequence of development can be significantly extended by spectral environments which provide a highly biased stimulation of the two cone mechanisms.