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¹ University of Ilorin, Department of Zoology, Pmb 1515, Ilorin, Nigeria Viewed : 9280 - Downloaded : 14164 The variations in selected physico-chemical factors were investigated for two years to determine the water quality of Oyun Reservoir, Offa, Kwara State, Nigeria for drinking and fish production. Three stations were chosen on the reservoir to reflect the effect of human activities, lacustrine and lotic habitats. Temperature, transparency, pH, conductivity, total dissolved solids, dissolved oxygen, nitrate, phosphate, chemical oxygen demand, total alkalinity, total hardness, calcium, magnesium, silica, sulphate, carbon dioxide were analyzed monthly between January 2002 and December 2003 using standard methods and procedures. The ranges of these factors were found to be comparable to those reported for other African reservoirs except for nitrogen and phosphate which were found in higher concentration above freshwater limits. Runoff of nitro-phosphate and sulphate fertilizers from nearby farm lands and washing of cows dungs from the watershed into the reservoir were found to have caused cultural eutrophication in the reservoir. The eutrophication was pronounced at Station 1 due to impact of human activities on the watershed, and with time, it will affect the water quality and fish production in the reservoir. The study concludes that Oyun reservoir has excellent water quality, high ecological status and passes chemical status. Eutrophication which was noticed to be a threat to the water quality should be arrested at the nick of time through denitrification and nutrient control to halt the degradation of the water. Keywords : Eutrophication, Fertilizers, Ecology, Total Dissolved Solids, Fish production