Mass mortality of fish due to pesticide exposure is rare, and results only from accidents or direct spraying of the water bodies. More commonly, fish are subjected to long-term stress arising from exposure to sublethal concentrations. In the long run, these sublethal concentrations may prove more deleterious than the lethal concentrations, because subtle and small effects on the fish may alter their behavior, feeding habits, position in the school, reproductive success, etc. Behavioral or morphological changes may make the fish more conspicuous in the environment and more susceptible to predation or parasitization, thereby reducing the ability of the population to survive and reproduce. Likewise, subtle effects at the organ or cellular level may alter the metabolism of the fish and hence its ability to withstand stress. Even if the fish is not directly affected, any effect on fish-food organisms may result in a starved population of fish. In this chapter such effects of pesticides on fish are discussed.