The Unionidae family is the most abundant family among freshwater bivalves which belongs to the Unionida order. Considering the nutritional, economic and environmental importance of bivalve molluscs, as well as the lack of information related to freshwater mussels of the Unionida order, especially the species belonging to the Unionidae family, as well as their global risk and concidering the importance of the ovary and the testis as an important organ of reproduction and survival and since histological studies are the most accurate method in determining the stages of the gonadal cycle, this research was conducted with the aim of investigating the histology of oogenesis and spermatogenesis in Unio tigridis of the Karun River. Sampling was done seasonally. For this purpose, the studied species after catching in every season were transferred to the histological laboratory, then they were biometrically identified using valid identification books and keys. After dissection, small pieces of gonads were separated and after tissue passage, they were examined histologically and histometrically. The results of the histological studies of the gonads showed that this species is often heterosexual. The results of the histological studies of gonads showed that out of 41 oysters in this study, there was only one hermaphrodite, and testicular and ovarian tissue were simultaneously observed in the gonad tissue. The ovary was asynchronous type because, at any time, all types of follicles in different stages of development (oogonia, previtellogenic oocytes, vitellogenic oocytes) were visible in the ovaries. The highest rate of development and maturation of both male and female gonads was observed in spring. It seems that the main breeding time of Unio tigridis species in the Karun River in the study area is in spring.