The migration of epithelial cells during fibergenesis in the lenses of young rats was determined by tritiated thymidine autoradiography. Cells initially labeled in the germinative zone appeared in the lens bow within seven days. Irradiation with either 600 or 2,400 rads of x-rays did not greatly alter the migration rate, but it did interfere with normal differentiation and proper fiber deposition, as manifested by a displacement posteriorly of the lens bow nuclei and an accumulation of abnormal nucleated fibers in the posterior part of the cortex. The presence of labeled cells in those regions prior to the onset of opacification is consistent with the possibility of direct epithelial cell involvement in radiation cataractogenesis.