Abstract: The development of the ovary and the electronic microscopy of the structure ofmature ovum of the fish, Colossoma brachypomum Cuvier were studied. The ovary in those fish under two-year old remains at Stage I, and at Stage II for those fish between 2 and 3 years old. The female fish reaches sexual maturity above three years old, During Stages IV, V, VI, the oocytes are mainly in phases 4 and 5 with a few in phases 2 and 3. It is evident that the oocyte development of the fish is continuous and the repreduction of the females may be manipulated artificially many times within a year. The changes in the structure of oocytes during the development were also examined. It is further suggested that C. brachypomum belongs to the type of poly-spawning fish.