Black flies, vectors of Onchocerca volvulus, are known to colonize fast flowing sections of rivers. A study on aspects of ecology of immature forms of black flies was conducted at River Mada. Larvae and Pupae of the Genus: Simulium were prospected on natural substrates found at various water currents along the course of the river over a 12-month period. Larvae of six species and pupae of three species were identified colonizing different substrates in the river. A total of 443 larvae and 142 pupae were collected, and the peak month of collection was April. 405 (91.4%) larvae were identified as S. damnosum sl while 38 (8.6%) comprised S. adersi, S. hargreavesi, S. cervicornotum, S. bovis and S. vorax. 110 (77.5%) of the pupae collected were S. damnosum sl while 32 (22.5%) were S. adersi, and S. hargreavesi. The larvae unlike pupae appeared to be non discriminatory in substrate colonization and current preference. Paucity of the population of immature forms is observed and attributed primarily to scarcity of suitable substrates and other factors also discussed herein.