Larvae of the clawed toad, Xenopus laevis, were thymectomized at the Nieuwkoop-Faber stages 45–47 (4- to 7-day-old) and 51 (18-day-old), and raised beyond metamorphosis to study their immune reactivities. The toads thymectomized at stage 51 and the unoperated and sham-operated controls rejected allografted skins in 16–37 days. In contrast, the toads thymectomized at stages 45–47 did not reject the grafts at all, and there was no lymphoid invasion into the grafted area. The toads thymectomized at stage 51 produced antibodies against rabbit red blood cells (RRBC), as did the controls. However, those thymectomized at stages 45–47 did not produce specific antibodies. Electrophoresis on cellulose acetate membrane and immunoelectrophoretic analyses proved that in spite of the total lack of observed immune responses, the thymectomized toads do possess a certain level of immunoglobulins. It is concluded that the thymus exerts its decisive role in the establishment of immune competence when it is still in the initial state of differentiation. In relation to the immunoglobulins found in the early-thymectomized toads, a discussion was also offered of the presence of a thymus-independent humoral immune system.