Histochemical and electron microscopic studies of the vitellaria of three polyopisthocotylean fish-gill flukes,Diplozoon paradoxum, Diclidophora merlangi andD. denticulata, and one monopisthocotylean fish-skin fluke,Calicotyle kroyeri have shown that, in each case, vitelline cell development is basically similar. Immature vitelline cells are typically embryonic and undifferentiated. InD. paradoxum they display annulate lamellae, and inC. kroyeri they are invested with flat, sheet-like extensions of the parenchyma. 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. It is then released into the vitelline ductlet. These processes occur in all the follicles at the same time, and all stages of development can be found in any one follicle.