Abstract: Oogenesis and oocyte vitellogenesis in the flathead lobster Thenus orientalis were observed in detail by transmission electron microscopy.According to the shape and size of ootids, ultrastructural changes of nucleus and cytoplasm, vitelline accumulation and the relationship between oocytes and follicle cells, the oogenesis of T.orientalis could be divided into four stages, i.e.oogonia, previtellogenic oocytes, vitellogenic oocytes and mature oocytes.Besides a large and oval nucleus, the oogonia mainly contain vesicles of smooth endoplasmic reticula.The nucleus of previtellogenic oocyte had becomemarkedly swollen, and was named specially as germinal vesicle.The nucleolar exudates appeared in the cytoplasm near out nuclear envelope of previte-l logenic oocyte.The oocyte in vitellogenic stage was surrounded progressively by follicle cells and formed a follicle.More and more yolk granules were formed and accumulated in the cytoplasm of vitellogenic oocyte.In T.orientalis, vitellogenesis appeared to involve both endogenous and exogenous yolk formation.The free ribosomes were the first organelles involved in the endogenous yolk formation.And the dense proteinous part icles synthesized by these free ribosomes could be accumulated and developed d-i rectly in ooplasm into yolk granules without membranes.The rough endoplasmic ret icula were the main organelles involved in the endogenous yolk formation.The smooth endoplasmic reticula took part in the endogenous yolk format ion by being transformed to rough endoplasmic reticula or fusing with rough endoplasmic reticula.By various ways, mitochondria and lysomes were involved act ively in the formation of yolk granules.In addition, golgi vesicles were also involved in the yolk format ion.There were two sources, the synthetic product of follicle cells and the precursor of lipovitellin carried by haemolymph, for exogenous materials accumulated in the perivitelline space.These exogenous materials were uptaken into oocytes by the micropinocytosis of plasmalemma and the absorption of microvilli, which were the two main ways of exogenous yolk formation.When oocyte became mature, ooplasm was nearly filled with yolk granules.Both endogenous and exogenous yolk materials were involved in the format ion of yolk granules with myelin figures in mature oocytes of T.orientalis.The formation of chorion in perivitelline space and the retraction of microvilli from chorion and perivitelline space could be regarded as the morphological markers of mature oocyte in T.orientalis.