This paper describes the distribution of thymus-derived lymphocytes in the spleen of the lizard, Calotes versicolor. Our studies reveal the existence of red and white pulp in the spleen. The red pulp consists of collagenous fibrous septae containing lymphocytes, erythrocytes and sinuses. The white pulp is an area enclosed by fibrous septae, containing lymphocytes, reticular cells and arterioles. The region around the arteriole in the white pulp was depleted of lymphocytes either one month after adult thymectomy or after anti-thymocyte serum treatment. The repopulation of this region was observed in sham-thymectomized controls, but not in thymectomized lizards. Therefore, this peri-arteriolar region may be designated as a thymus-dependent area, as described previously in higher vertebrates. The significance of these findings is discussed in relation to the phylogeny of the immune system.