The Ogun River is a sink for untreated effluents discharged from the Kara Cow Market, Ogun State. In this study, microbiological, physicochemical and heavy metals evaluation were carried out on effluent from the market. Standard methods were utilised for the examination of physicochemical and microbiological parameters of the effluent. The histological effects of sublethal concentrations of the effluent were assessed in the gills, intestine and skin of Poecilia reticulata (guppy fish) over a period of 56 days following standard methods and international ethical guidelines. None of the physicochemical parameters were within the set limits by the National Environmental Standards and Enforcement Agency (NESREA) for effluent discharges into surface waters. Coliforms, pathogens and fungi were observed in the effluent with a total plate count of 2.14 x 10⁷ cfu/mL. The median lethal concentration (96 h LC₅₀) of the effluent to P. reticulata was 71.50 mL/L (7.15%). Histological alterations such as distorted mucosal architecture with shortening and widening of intestinal villi were observed in the intestine of exposed groups of P. reticulata at day 56. The observed adverse physicochemical parameters, histological alterations and pathogenic microbes may pose ecological and public health risks to aquatic organisms and humans respectively. It is recommended that an effluent treatment plant should be installed at the market to reduce the pollutants load of the effluent before discharge into the Ogun River.