Abstract: An Electron microscopic studies of the vitellaria of three polyopisthocotylean fish-gill flukes,Diplozoon paradoxum, Diclidophora merlangi andD. Denticulata are composed of cells in different stage of development.(Halton et al 1974) Immature cells are embryonic and undifferentiated. Differentiation into maturing vitelline cells involves the development of extensive GER, Golgi complexes and the production of dense droplets of shell-protein. With the onset of maturity, protein synthesis stops and, as the GER disintegrates, the cell develops food-reserves in the form of yolk bodies, glycogen and lipid and released in the ciliated vitellian duct. Vitelline development is continuous and all of the cellular stages involved can be found in each follicle.