Simulium damnosum (Theobald) sensu lato (s.l.) is a complex of many species of black flies that transmit Onchocerca volvulus (Leuckart) to varying capacities based on their ecological zones in Africa. The presence of three ecological zones in Osun, an onchocerciasis endemic state in Nigeria, is the basis of this study that is aimed at determining the population dynamics of S. damnosum s.l. in the state. Adult S. damnosum s.l. were collected fortnightly in the wet and dry seasons for 2 yr between October 2014 and September 2016 in the Guinea savanna (Iwo), derived savannah (Ede), and rainforest (Obokun) zones. Temperature, relative humidity, and rainfall of the study area were measured. The results showed that in the first year, Ede had 62.8% of the total black fly population while Obokun had the lowest (1.5%). In the second year, Iwo had 94.1% of the total black fly population while Obokun had the least population.The black fly population was significantly higher during the wet season than dry season in Iwo and Ede, but was not significant in Obokun. The results further showed that black fly populations were strongly correlated with ambient temperature and rainfall in Iwo, whereas no relationships were recorded for Ede and Obokun. The results suggest that abundance of black flies during the wet season was due to increased rainfall which in turn created rapids and conditions suitable for development of preimaginal stages into adults.These vector surveillance findings will guide control decisions necessary for endemic communities to meet elimination targets.