Larvae of Xenopus laevis at stages 50-53 were lentectomized and then injected with tritiated thymidine at various times after lentectomy. In series I, the animals were injected 1,2,3,4,6,8,10,12,15, or 17 days after lens removal and fixed three hours after injection. Autoradiograpns of serial cross sections through the eyes were prepared. Increased incorporation of thymidine in the cells of the regenerating lens was first observed two days (stage 2) after lentectomy. All of the cells of the lens vesicle incorporated H3 thymidine until stage 4 at which time the cells in the inner wall of the vesicle began to differentiate into lens fibers. Labelling then became restricted to the peripheral cells (prospective lens epithelium and prospective lens fibers). At stage 5 of regeneration, only cells of the lens epithelium incorporated H3 thymidine. In Series II, animals injected three or four days after lentectomy were fixed daily from one to seven days after injection. Many stage 3 and stage 4 regenerates were recovered with label throughout and stage 5 regenerates were found, seven days after injection, with label over the lens fibers as well as over the lens epithelium.