Abstract: The constituents and bacterial population of an oilfield wastewater and tissues of Clarias gariepinus exposed to sub-lethal concentrations of the oilfield wastewater were investigated. Some physicochemical parameters, total heterotrophic and petroleum degrading bacterial counts of the wastewater and tissues were determined using standard methods. The mean values of physicochemical parameters obtained were: Temperature 25.93±6.7°C, pH 7.73±0.31, turbidity 40.33±1.53 NTU, salinity 6584±137 mg L-1, conductivity 15200±1058.68 μS cm-1, Total Dissolved Solids (TDS) 8436.33±501.68 mg L-1, Total Suspended Solids (TSS) 4.67±0.58 mg L-1, chloride 4033.37±208.17 mg L-1, alkalinity 1296.33±2168 mg L-1, Dissolved Oxygen (DO) 1.83±0.38 mg L-1, Biochemical Oxygen Demand (BOD) 1.3±0.7 mg L-1 and Total hydrocarbon (THC) 40.54±50 mg L-1. The values for TDS, salinity, conductivity and alkalinity were greater than FEPA limits while BOD and DO values were lower. Values of other constituents were within the acceptable limits of FEPA. Mean total heterotrophic counts and petroleum degraders in the oilfield wastewater were 1.59±0.57x108 CFU mL-1 and 42.1±17.4%, respectively. Bacterial counts for the tissues of Clarias ranged from 0.04±0.01x106 to 0.29±0.001x106 CFU g-1 for the skin, 2.49±0.35x106 to 4.15±0.13x106 CFU g-1 for the gills and 1.22±0.00x106 to 3.15±0.24x106 CFU g-1 for the intestine. The gills had higher bacterial counts. The bacteria isolated included Alcaligenes, Bacillus spp., Chromobacterium, Enterobacter spp., Escherichia coli, Klebsiella, Micrococcus, Proteus, Pseudomonas fluorescens, Serratia spp. and Staphylococcus aureus. The highest occurring bacteria were Staphylococcus aureus (80%), followed by E. coli (55%) and Enterobacter spp. (50%) while the least was Chromobacterium (5%) that occurred only in the oilfield wastewater. All bacteria except E. coli and Chromobacterium were isolated from the wastewater and C. gariepinus, respectively. The high levels of physicochemical constituents and very low DO of the oilfield wastewater is a hazard to fish if discharged into water bodies. The presence of potential pathogens such as Bacillus, E. coli and Staphylococcus among others can lead to bacterial diseases of fish, economic loss and public health hazards. The proper treatment of oilfield wastewater prior to discharge into the recipient water body is advocated as to reduce ecotoxicological problems.