Fish thymus is phylogenetically the first well..developed lympho..epithelial structure for most fish species. It supplies microenvironment for T..cell differentiation and is directly involved in defense mechanism. The current status of the f ish thymus research was reviewed in the fo llowing aspect s: 1) structure and possible funct ions; 2) origin, histogenesis and degeneration; 3) ontogeny and function of T..lymphocyte obtained with monoclonal antibody.