A survey was carried out on species composition, abundance and seasonal distributions of fish fauna in Ishasi area of Lower Ogun River from January 2006 to December 2007. Fish samples were collected using cast net and drop traps on monthly basis from three stations (stations 1, 2 and 3). Relative uniform distribution of fish fauna was observed as fish abundance and diversity levels recorded from the three stations were not significantly different (p>0.05). A total of 1419 specimens of fish of forty-three species and seventeen families were collected with Margalef species richness and Shanon-Wiener diversity indices values of 7.79 and 2.76 respectively. Barboides gracilis of Cyprinidae family dominated the fish species with relative abundance of 21.42% followed by Tilapia zilli, a Cichlid (12.36%) and Aphyosemion gulare, a Cyprinodontid (9.66%). Species collected also included endangered single-species-in-the-family such as Lates niloticus (Centropomidae) and Heterotis niloticus (Osteoglossidae). In term of family, Cichlidae was the dominant with seven species representation followed by Characidae with five species. The rainy season had higher fish total abundance (1061), species number (38), and diversity levels than dry season when a total abundance of 358 and 25 species were recorded. The imperativeness of sustainable management and conservation of the fish resources in Lower Ogun River is discussed.