In the teleost fish medaka, an adult ovary simultaneously contains developing oocytes at all phases of oogenesis during the breeding season. However, it remains unclear where oocytes at each developmental stage are located in the ovary by the time of ovulation. To examine the relationship between the developmental stage of oocytes and their positions in the ovary of vertebrate medaka during oogenesis, the stage of oocyte development was determined from the diameter of the oocytes and the cellular morphological characteristics, such as the germinal vesicle and micropyle at the animal pole and attaching filaments at the vegetal pole, and the positions of developing oocytes in the ovary in all sections were observed. Furthermore, to investigate the characteristics of the dorsal ovarian epithelium to which the oocytes attach themselves, the dorsal and vegetal ovarian epithelia were observed. The dorsal ovarian epithelium invaginated in spots. When all serial sections of the oocytes were observed, all oocytes at stages I–VIII were attached to the dorsal ovarian epithelium, regardless of whether it invaginated or not.