To study how different visual deprivation conditions influence the degree of myopia, chicks were reared under conditions of unilateral and bilateral visual deprivation with translucent and black goggles in environments of a 12-hour light/dark cycle and complete darkness. Two weeks after hatching, the refractive errors and axial lengths of the eyes were measured. The deprived eyes of the light/dark-reared chicks exhibited severe myopia both unilaterally and bilaterally as well as axial elongations. The dark-reared chicks exhibited slight myopia in the eyes with goggles and no significant axial elongation. The experiments suggest that different degrees of visual deprivation can also cause severe myopia, that myopia develops independently in each eye, and that a light/dark environment is required for severe myopia to develop.