This study aimed at inspecting pollution impact on the reproduction process of Tilapia zillii (Gerv.) living in three main basins of Lake Mariut. Our results revealed that there were two main seasons detected for males and females during a whole year of investigation, namely, non-spawning and spawning seasons. In males, though the two seasons started and ended simultaneously, gonadosomatic index values of males living in the relatively clean area were significantly higher than those of the other two polluted areas. However, no structural alterations could be detected in sections of male gonads from the three basins.  All stages of maturation were represented in all gonad sections. In females, gonadosomatic index values of fish living in the relatively clean basin were significantly higher than those of fish living in the other two polluted areas. Females living in the relatively clean area attained two prominent peaks during the spawning season compared to one peak for the other two polluted areas.  Spawning season for one of the two polluted areas was one month shorter than the reference area. Histologically, belated and deformed maturation stages as well as ruptured and empty follicles were observed in female gonad sections from the two polluted areas.