Effects of Diesel on Fresh water fish (Clarias gariepinus) was carried out in the Fish Pathology Laboratory of the Department of Fisheries and Aquaculture, University of Calabar, Nigeria. About 200 fish were purchased from the University Fish farm, acclimated for 48h in the laboratory. The diesel used in this experiment was obtained from the University of Calabar power generating unit. Experiment was conducted in plastic containers of 10l capacity, each containing 10 fish and 5l of diesel in different concentrations (0.25, 0.37, 0.50 and 0.75g/l). The control tanks had similar number of fish and same volume of water without diesel. The tests and control tanks were replicated three times under same laboratory conditions. The duration of the experiment was 96h with hourly observations for abnormal fish behavior and mortality. Erratic swimming behavior, fast opercula movement and hanging on the water column and weakness were observed in test tanks only whereas fish in the control were calm and normal. Mortality of fish (3, 5, 7 and10) occurred in the test concentrations respectively within the duration of the experiment. Histopathology of gills of affected fish showed damaged gill lamellae, similar changes were not observed in the histological sections of the control fish. Mortality data obtained were analyzed graphically by plotting % fish mortality against log concentrations of diesel. The concentration of diesel in which 50% fish mortality (LC50) occurred was 0.37g/l. The results of the experiment led to the conclusion that diesel at different concentrations has deleterious effects on fresh water fish (C. gariepinus) and could cause mortality on repeated exposure to the organisms at higher concentrations. Fresh or used diesel should not be discarded into the environment to safeguard freshwater fish and other organisms. Further study on the effects of diesel on the food chain is recommended.