The reptile immune response is performed by a well-developed immune system whose leukocytes have been characterized as lymphocytes, monocytes and granulocytes. A main characteristic of the reptile immune system is the effect of the seasonal cycle on its histology and function, as has been reported in the turtle Mauremys caspica. In the present study, we have assessed the presence of lymphocytes in the main lymphoid tissues (blood, spleen, and thymus) searching for putative seasonal variations and sex-associated differences in lymphoid distribution. Our results show that lymphocytes are the most commonly observed leukocyte subpopulation in blood. The highest percentages of blood lymphocytes were obtained in spring. Lymphoid tissues showed higher proportions of lymphocytes in spring and summer. Finally, females showed higher lymphocyte proportions than males in the spleen during spring and in the thymus during summer. Summarizing, the lymphoid redistribution of M. caspica is affected by the seasonal cycle. The major proportion of lymphocytes was obtained in spring and summer; the seasonal periods when these turtles are more active, and the risk of infections is higher.