Abstract: According to the previous studies,snake (Rhabdohpis tigrina) thymus is also a neuroendocrine organ.The existence of APUD cells in the immune organs and their properties were observed in 11 species of different vertebrate groups from fish to mammal with special staining techniques and immunocytochemical methods in this paper.The results showed that there were some argyrophil,argentaffin,lead-haemutoxylin positive and 5-HT,Gas or SS-immunostaining cells in thymus of frogs (Rana nigromaculata,Kaloula borealis),toads (Bufo bufo gargarizans,Xenopus laevis),chicken (Gallus gallus domestica) and Peking duck (Anas platyrhynchos domestica).This is the same as in the snake,but in the other immune organs.According to the immunostaining result,there was a difference in type and number of positive cell between the species.The result indicated that thymus had neuroendocrine function and was a neuroendocrine organ from the evolutionary point.