A cytotaxonomic study was carried out to determine the species composition and dynamics of Simulium damnosum sensu lato in Oyan River, an onchocerciasis mesoendemic area in southwest Nigeria. Simulium larvae were collected fortnightly from different attachments between March and November 2012 fixed in Carnoy's solution for laboratory identification using cytotaxonomic keys. A total of 255 Simulium larvae were obtained and identified from their polytene chromosomes. Simulium soubrense Beffa (56.5 %) and Simulium damnosum sensu stricto (43.5 %) were the two cytospecies observed. No new inversions were found in the larvae. Statistical analysis revealed a significant difference (P<0.05) in the abundance of S. damnosum s.s. and S. soubrense in relation to rainfall pattern. The sympatric breeding of both species in the area may suggest their involvement in the transmission of onchocerciasis in the area, as S. damnosum s.s. and S. soubrense Beffa are known vectors of Onchocerca volvulus in West Africa.