The refractive error observed in experimentally-induced myopia in chicks appears to occur as a result of expansion of the vitreous chamber. Fourteen days after the application of a goggle to one eye, the eye was myopic and had a greater volume of liquid vitreous than the non-myopic eye. The relative proportion of liquid to gel vitreous was also higher in the myopic eye than in the non-myopic eye. The results suggest that the vitreous humor plays a key role in the development of experimentally-induced myopia in chicks.