Under scanning electron microscope ( SEM ) , the surface microstructure on lymphocytes of Ctenopharyngodon idellus is characterized with full of bumpiness and holes, or rather smooth or protruding with small cones, whereas the surface microstructure on lymphocytes and thymocytes of Trionyx sinensis is characterized w ith full of bumpiness and holes, or rather smooth. The lymphocytes and thymocytes f illed with bumpiness and holes are evidently more than others. Under optic and scanning electron microscope, the rosette format ion of lymphocyte of C. idellus with SRBC is not obvious, whereas rosette forming percents in lymphocytes and thymocytes of T. sinensis are respectively 25% - 34% and 36%- 47% . The lymphocytes and thymocytes which form rosettes w ith SRBC are predominantly ones f illed with bumpiness and holes ( similar to T lymphocyte of mammalian) . The immuno-l histochemistry on the cross-reactivity by using monoclonal antibody against human CD2 shows that the CD2-positive percent of lymphocytes in peripheral blood and thymocytes of T. sinensis are respect ively 24% and 33% , but no cross-react ivity is found on lymphocytes in peripheral blood of C . idellus.