We performed structural and immunohistochemical studies on the thymus of juvenile turbot (Psetta maxima L.) in order to define its cellular composition. The thymic stroma was mainly composed of two subpopulations of reticulo-epithelial cells (RECs). RECs immunoreactive to anti-actin antibody were distributed through the organ, while RECs that were cytokeratin-immunopositive were located in the outer zone of the thymus. The parenchyma of the thymus was composed of several cell types such as lymphocytes/thymocytes, lymphoblasts, melano-macrophages and to a lesser extent of nurse-like cells, immunoglobulin positive (Ig+) cells, mucous cells, rodlet cells and neuroendocrine cells. CD3ε+ lymphocytes were mainly located in the outer zone. On the other hand, Ig+ cells were observed in the transitional region between the inner and outer zones of the thymus. The neuroendocrine cells were large and exhibited immunoreactivity to neuropeptide Y and vasoactive intestinal polypeptide. They were located in the inner zone of the thymus in close association with lymphoblasts and lymphocytes/thymocytes. This work provides useful information on the structure and cellular composition of the thymus of turbot, identifying several immunomarkers that allow the identification of different cell types, providing the basis for further studies on the immune response of turbot against diseases.