The vitellogenesis of the oocytes from the Chinese mitten-Tianded crab, Erlocheir sinensis, was examined by light and electron microscope. The results show that previtellogenic oocy1es belong to prirnary oocytes in a slow developrnent. Each oocyte is surrounded by a 1ayer of follicle cells. The surface of the oolemma show no specializations and is smoth. The ooplasm, containing no yolk granules,possesses the basophilic property.The numbers of the organelles such as endoplasmic reticulum are much greater than in the oogoniuru. One to two condensed nucleolus are prcsent in the nucleus. The vitellogenic oocytes grow fast and their volurne increase significantly. Lipid vesicles and numerious yolk granules appear throughout the ooplasm. The endoplasmlc reticulum, Golgi complex,ribosornes and lysosnles are involves in the formation of yolk granuios. The nucleus become germinal vesicle in which one to threc descontlensed nucleolus are seen. The surface of oocytes forru a few microvilli that proJect into periv1telline space in which electron-dense material are present, but no absorbed by the oocytes. The postvitellogemic oocytes contain a fewer numbers of the organelles. The intra-oocytic synthesis are modified. The layer of follicle cells become very thin. The perivitelline spdce fills with electron-dense material that are ingested by the oocytes by pinocytosis.