Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4&lt
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and CD8α&lt
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T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4&lt
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helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α&lt
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, CD4&lt
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T-cells and sIgM&lt
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cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.