Residues of agrochemicals used in intensive wetland farming are discharged into the aquatic environment through runoff or wind drift thus affecting aquatic life. Effects of these agrochemicals on fish resources of wetland environment should be studied. This study was conducted to establish baseline information on major agrochemicals in use for wetland farming in Oyo State and to examine their impact on fish diversity.The experiment was carried out in two stages. Purposive sampling of all the eleven Local Government Areas (LGAs) of Oyo State involved in the Federal/World Bank assisted wetland-farming project was carried out. Seasonal samples of water, soil and fish in the selected locations were also collected according to standard methods to investigate possible agrochemical pollution. The second stage involved bioassay test. The data obtained were analyzed using t-test and analysis of variance (ANOVA). The study showed that ecological consequences of uncontrolled usage of agrochemicals have led to water pollution and leading to reduction in fish biodiversity in the study areas.