Seasonal reproductive cycle, gonad structure, gametogenesis, larval development and sex ratio of the freshwater mussel, Nitia teretiuscula (Philippi, 1847) was investigated for the first time. A total of 200 mussels, were randomly collected each month from September 2016 to August 2017 from four regions along Nile River, Egypt: El-Kanater (Qaluobiya Governorate), Tura (Helwan Governorate) Geziret Bein El-Bahrein (Giza Governorate) and Geziret El-Dahab (Cairo Governorate). Histological examination of the gonads showed that N. teretiuscula is a dioecious species with a 1:1 sex ratio. The female and male gonads were found to be paired and asymmetrical organs, situated between the mantle and digestive gland. Female follicles contain edoogonia and oocytes that could be separated into, previtelogenin, vitelogenin and postvitelogenin stages. Male follicles contained spermatogonia and spermatocytes that could be classified as primary spermatocytes, secondary spermatocytes and spermatids. Moreover, the reproductive cycle of both sexes included four developmental stages, developing, ripened (mature), spawning and spent. The predominant stages were ripening (mature) and spawning, whereas spent gonads were rare. Gametogenesis began in early spring and spawning occurred in late spring. Although mature oocytes and spermatozoa were present for almost the entire year, gonadal activity seemed lowest in autumn months. Both sexes displayed the most gonadal activity during the winter months. Consequently, eggs and larval stages (glochidia) were incubated for a short period within both the outer and inner demibranchsso; N. teretiuscula can be considered atetragenic and tachytachtic species.