Abstract: The present paper deals with two species belonging to parasitic copepoda of the family Ergasilidae.Two species of Neoergasilus have been observed for the first time by the scanning electron microscopy.It is discovered that the ultrastructure of swimming legs and the morphology of thorax-plate are different from each other:leg 1 exopod first segment outer spine,biforked in N.longispinosus,comb-like in N.japonicus;leg 2 and 3 endopod third segment spine,dichela-like in N.longispinosus,monoramose in N.japonicus;leg 2 to leg 4 exopod first segment outer spine,long in N.longispinosus,short in N.japonicus.These differences stated above are called the expression of differentiation between two species and those differences in whole biological kingdom are called the expression of biodiversity.