My basic aim is to develop an environmentally safe plant origin piscicides because they are supposed to be less expensive, easily biodegradable, easily available, easy to handle and safe for mankind and environment. So, I investigate two locally found medicinal plants under following sections: 1. Extractions of compounds from different plant parts of Euphorbia royleana and Nerium indicum, and test their piscicidal activity against common freshwater predatory fish Channa punctatus. 2. Effect of sub-lethal doses (40% and 80% of LC50) of extracted compounds (having maximum piscicidal activity), on fish metabolism. 3. Identification and structure elucidation of those extracted compounds. I have find five out of seventeen compounds which have maximum piscicidal activity and their sub-lethal doses altered behavioural pattern, inhibit oxidative metabolism of fish and suppress energy production by inhibiting ATP production, but this was advantageous that toxic action of these compounds were reversible, despite their high toxicity. So they can be used as a environmentally safe piscicide for controlling predatory fish Channa puctatus population from aquaculture ponds.