This study reports the first classification of cytological structures in developing gonads of cultivated Haliotis midae. Gonad tissue from animals ranging from 6–48 mo of age was evaluated histologically. The process of oogenesis was divided into 9 distinct stages according to the size and histological structures of each cell type, whereas spermatogenesis was divided into 5 distinct stages. Six-mo-old animals showed initial development of germinal epithelium, whereas at 12 mo of age, the gonad cavity was visible. In 18-mo-old males, spermatogonia and primary spermatocytes were identified, whereas in 24-mo-old males, all cell stages of spermatogenesis were present. Oogonia and stage 1 oocytes were seen in the gonad cavity of an 18-mo-old female, whereas in the gonad of 2-y-old females, all stages of oogenesis could be distinguished, although mature oocytes were not abundant.