The oogenesis of the Crustacea Amphipoda Orchestia gammarellus can be divided in five stages taking into consideration both the oocyte ultrastructure and the physiology of the ovary. The primary oogonium (12 micron in diameter) is lodged within the germinative zone: after division, the daughter cell (or secondary oogonium) leaves this area and enters meiotic prophase. Stage I is represented by the oocyte with visible chromosomes (12-18 micron in diameter) the cytoplasmic ultrastructure of which is comparable to that of the oogonium. Stage II or previtellogenesis is characterized by a considerable growth of the oocyte (18-80 micron in diameter) which becomes enriched in ribosomes and vesicles of the rough endoplasmic reticulum; the oocyte does not yet contain any vitelline reserve (proteinaceous and lipid). Stage III or primary vitellogenesis (80-160 micron in diameter) is typified by the synthetic activity of the rough endoplasmic reticulum, corresponding to an endogenous accumulation of proteinaceous yolk. Stage IV or secondary vitellogenesis (160-800 micron in diameter) only appears during the period of reproduction; by means of endocytosis the oocyte accumulates yolk spheres in addition to lipid droplets, the origin of which is uncertain; towards the end of vitellogenesis, cortical granules become a feature that is noted for the first time in Crustacea. The last stage or maturation (800 micron in diameter) starts right before or immediately after the exuviation of the female and ends with fertilization.