Abstract: This paper deals with the development of the oocytes in the ovary of the sexually mature pond-cultured Grass carp. During the years 1979—1982, the ovarian development of 80 specimens of 4—5 years old has been studied. These specimens were collected in May, August, November and the next January every year. The results show that the histological structure of the ovaries before spawning time, contains the oocytes of phase 2, 3 and 4 with the phase 4 occupying the maximum, part of the ovary, while phase 2 and 3, the minimum. After spawning, the oocytes of phase 2 and 3 increased and phase 4 decreased immediately. The number of the phase 4 oocytes increased again in August; they occupied a volume of 80% of the ovary and then decreased in November. From November to next January, the oocytes of phase 3 increased and phase 4 disappeared from the ovary. During Feburary, the new phase 4 oocytes appeared and increased gradually till the spawning time. The ovary underwent a series of elaborate and gradual changes which eventually led to the formation of the mature sex cells. The oocytes do not develop synchronously from one phase to the next, but a large number of the oocytes developed from phase 3 to phase 4 before spawning time, and a great number of phase 2 developed in ovary after spawning. The results also show that the histological structure of the ovary of the Grass carp possesses the biological basis of spawning twice in one year cycle, but the fish spawns only once a year in most parts of the area south of the Changjiang river, due to certain unfavorable environmental factors.This paper also shows that the oocytes of phase 2 can be subdivided into early and late stages based on their cytological characteristics.