This book deals chemical investigation of two regional, medicinal plant Euphorbia royleana (Family - Euphorbiaceae) and Nerium indicum (Family - Apocynaceae) with a view to isolate and characterise their chemical constituents which is responsible for their piscicidal activity against freshwater air breathing predatory fish Channa punctatus and biochemical effects of these constituents were also under taken, because they cannot be used as environmentally safe plant origin piscicides without a study of these aspect as well. In present study, seventeen compounds were extracted and their piscicidal activities were tested, among of them only five compounds (two from E. royleana and three from N. indicum) have optimum piscicidal activities against fish Channa punctatus. After exposure to sub-lethal doses of these compounds, respiratory pathway of fish shifts from aerobic to anaerobic segment. But it is beneficial that not only the mode of action of these compounds were reversible in their action but also their toxicity persist short duration inside the aquatic environment. In consequence, these compounds can be used for controlling predatory fishes in aquatic environment without harm.