The Mycological flora of Clarias gariepinus exposed to sub-lethal concentrations of an oilfield wastewater were investigated. The concentrations included 0% (control), 10, 20, 30, 40, 50 and 60% respectively. Physico-chemistry and mycoflora of wastewater and tissues of Clarias gariepinus were determined using standard methods. Mean values obtained were; temperature 25.93±6.7oC, pH 7.73±0.31, turbidity 40.33±1.53 NTU, salinity 6584±137mg/l, conductivity 15200±1058.68µs/cm, total dissolved solids 8436.33±501.68mg/l, total suspended solids 4.67±0.58mg/l, alkalinity 1296.33±2168mg/l, dissolved oxygen 1.83±0.38mg/l, biochemical oxygen demand 1.3±0.7mg/l and Total hydrocarbon 40.54±50mg/l. Temperature, DO, BOD and THC were below allowable FEPA limits while all other components were higher. Mean counts of total fungi and petroleum degraders in the oilfield wastewater were 4.7±0.46×106 sfu/ml and 59.7±25.7% respectively. Fungal counts in the tissues of Clarias ranged from 0.20±0.00 x 104sfu/g to 3.00±0.00 x 104sfu/g (skin), 0.48±0.05 x 104sfu/g to 7.25±0.96 x 104sfu/g (gills), and 1.13±0.15 x 104sfu/g to 5.75±0.50 x 104sfu/g (intestine). The intestine had higher fungal counts, but the gills recorded the highest at 10% concentration. Fungi isolated included; Aspergillus fumigatus (46.43%), Aspergillus niger (100%), Fusarium spp. (100%), Mucor spp. (24.99%), Penicillium spp. (57.14%), Rhizopus spp. (32.13%) and Saccharomyces spp. (34.3%). All except Saccharomyces spp were isolated from oilfield wastewater. Aspergillus spp. Penicillium spp, Mucor and Rhizopus are considered normal flora, but can still causeinfection which may result in the mortality of the fish and eventually economic loss to the aquarium fish industry. Proper treatment of oilfield wastewater prior to discharge into the recipient water body is advocated to reduce ecotoxicological problems.