There is a clarion call to protect the environment from pollutants and toxins. Besides habitat loss and over exploitation, pollution is ranked as the main cause of fish species loss and the use of pesticides is one of the main causes of aquatic pollution. A lot of factors like increased agricultural activity and industrialization due to the increase population jointly contribute to the loading of the environment with residues of potentially hazardous pesticides and other toxic pollutants. Diazinon is a common active component of organophosphorus pesticide which is widely used as an insecticide, acaricide and nematocide; but has been found to be highly toxic to birds, mammals, honeybees and freshwater fish following an acute exposure. Therefore, there is a need for bioassay studies of pesticides effect on different fish species and offer possible recommendation to protect the environment.