Abstract: Yangtze sturgeon, Acipenser dabryanus, is a freshwater fish that mainly distributed in the upper reaches of Yangtze River and its tributaries. Male and female Yangtze sturgeons in the wild reach sexual maturity at 4—6 and 6—8 years, respectively. Currently, it is a critically endangered species due to overfishing, habitat degradation and pollution. In order to protect and restore this species, many efforts have been made, such as building nature reserves and fishery stock enhancement and releasing. However, the wild population of Yangtze sturgeon has sharply decreased, and it is hardly observed in the Yangtze River now. Previous studies, such as diet supplemented with exogenous hormone and juveniles injected with peptide, were performed to short its sexual maturation time, but no effect has been observed. The oogenesis-related gene, org, plays an important role in the growth and development of oocytes in teleost fish. In order to reveal its function during the oogenesis of Yangtze sturgeon, a full-length cDNA of an org homologue was isolated (designated as Adorg), which was 1031 bp, encoding 233 amino acids. Multiple sequence alignments showed that AdOrg shared the highest sequence identity (49.5%) with zebrafish. By quantitative real-time PCR analysis, Adorg mRNA was specifically transcribed in the gonad, abundant in the ovary and weak in the testis. Transcription of Adorg was not detected in other somatic tissues including liver, intestine, spleen, kidney, heart, muscle, gill, pituitary and hypothalamus. During embryogenesis, Adorg was proved to be maternally transcribed, maintaining a high level before the gastrula stage, and then declining dramatically in later developmental stages. The transcription of Adorg was very limited in undifferentiated gonads, but it sharply increased in the following process of oogenesis, with its highest expression in stage Ⅱ oocytes. In situ hybridization of gonad indicated that the signal of Adorg mRNA was specifically located in the germ cells. In the ovary, Adorg signal was weak in the oogonia, and it increased rapidly in the cytoplasm of primary oocytes, and became stronger with the development of oocytes. In the testes, the expression of Adorg was restricted to type A and B spermatogonia, and was barely detectable in spermatocytes. These findings suggested that Adorg gene might play vital roles not only in oogenesis, but also in development and differentiation of germ cells. Knock-down or knock-out of this gene is necessary for exploring its specific function. These results paved the way for understanding the function of Adorg gene in Yangtze sturgeon during oogenesis.