The adult ovary was examined in a freshwater crayfish, Procambarus clarkii, to clarify the ovarian structure and the mode of oogenesis. A Y-shaped ovary consisting of a pair of anterior ovarian sacs and a single posterior ovarian sac was located in the cephalothorax, on the dorsal side of the stomach. An oviduct connected each of the posterior ends of the paired anterior ovarian sacs with the genital pore on the coxa of the 6th appendage. The wall of the ovarian sacs, consisting of a layer of the ovarian epithelium, folded inwards to form a number of oogenetic pouches of various sizes. Each oogenetic pouch contained one egg or large oocyte, vitellogenic or previtellogenic, sometimes followed by a few early previtellogenic oocytes in the oogenetic pouch lumen. Germaria containing oogonia, very early previtellogenic oocytes and somatic interstitial cells were located in the ovarian epithelium near the bases of the oogenetic pouches. In a cross-section of the ovarian sac, the germaria were concentrated in the center of the ovarian sac as a central germarial cluster. An early previtellogenic oocyte beginning to grow left its germarium and raised the ovarian epithelium to form a new oogenetic pouch, in which it remained until mature. Mature eggs were ovulated from the oogenetic pouches into the central ovarian lumen, transferred into the oviducts, and oviposited through the genital pores. The female reproductive system was surrounded wholly and tightly by a thin muscular sheath, which has often been mistaken as the ovarian epithelium in some decapod crustaceans.