Diazinon is widely and effectively used in agriculture as well as in domestic and aquaculture aspects. As a result of its widespread application, its ecotoxicity study is needed to determine its effects on the aquatic organisms and
its potential impact as an environmental contaminant. In this study, the half lethal concentration (LC50) of diazinon for Clarias gariepinus was calculated. Diazinon toxicity was evaluated through acute and chronic exposures of fish to
1/2 and 1/10 the calculated LC50 for 1w & 6 w respectively. Results showed that the LC50 /96h was 5.98 ppm. The effects of both acute and chronic exposures revealed clinical abnormalities in the form of nervous and respiratory disorders manifested by erratic swimming and/or circling, rapid opercular movements and abnormal skin pigmentation that appeared in the form of skin fading. A significant increase in cortisol, glucose, AST & ALT activities, urea, uric acid,
creatinine and serum Ca++ levels were recorded. Furthermore, diazinon exposures were associated with an inhibition of cholinesterase activity, a decrease in total protein concentration in serum with a reduction in both albumin
and globulin. This study declared that diazinon is an extremely toxic organophosphate pesticide for C. gariepinus, its improper application often results in contamination of the environment and consequently in deleterious health effects. So, strict precautions should be followed.