Changes in the distribution of simuliid species and their disease transmission pattern may occur in response to climatic and environmental factors. Simuliid larvae were prospected in 34 rivers of Nasarawa State from January 2010 to December 2010. A total of 198,420 simuliid larvae collected were found to belong to six species as follows: S. damnosum s.l. (72.13%), S. adersi (24.31%), S. ruficorne (2.52%), S. cervicornutum (0.69%), S. alcocki (0.68%), and S. schoutedeni (0.17%). The monthly sample indicated continuous breeding throughout the year with peaks in July, August and September. The density of black fly larvae was estimated from samples collected on water plants per 1 dm2 of water plant surface. The study showed that both physicochemical and ecological parameters influenced the density of simuliids. Correlation analysis showed a significant relationship between simuliids density and the environmental factors. Data on prevalence and distribution of simuliids in Nasarawa State are sparse and scanty. Therefore, the result of this study provides additional information on the distribution and density of simuliids in Nasarawa State, Nigeria.