The sclera in myopic regions of chick eyes was studied histologically and compared to the sclera in corresponding regions of normal fellow eyes. Chicks had been monocularly deprived of form vision in the nasal half of the retina from hatching. The fellow control eye and the temporal retina of the deprived eye had normal vision. With this treatment, the resulting form-deprivation myopia and eye enlargement are restricted to the retinal region that had been form deprived. We found that the cartilaginous sclera in the myopic nasal region exhibited several differences from that in the corresponding non-myopic region: it was thicker, its cell density was lower, and the number of chondrocytes and binucleate cells was higher. In contrast, the fibrous sclera was thinner. These changes suggest that form-deprivation myopia causes an increased production of extracellular matrix and an increased level of mitotic activity in the cartilaginous sclera. As expected, the non-myopic temporal regions of experimental and control eyes did not differ in any of these parameters. The findings of the present study suggest that the eye enlargement accompanying form-deprivation myopia is not the consequence of scleral stretching but of abnormal growth.