Fish and other organisms are affected by pesticides which pollute the natural water through agricultural runoff. Fishes are common bioindicators of water pollution. In the present study bioassay of synthetic pesticide, Dichlorvos and a plant origin natural pesticide, Neem-On was separately done on Labeo rohita. Data obtained from the toxicity tests were evaluated using the Probit Analysis Statistical Method. The 96h LC₅₀ of Dichlorvos and Neem-On was found to be 16.71ppm, 42.66ppm respectively. The fish exhibited erratic swimming, copious mucus secretion, loss of equilibrium and hitting to the walls of test tank prior to mortality. In this study, Neem-On was less toxic to fish as compared to Dichlorvos. Plant based pesticides are biodegradable and are target specific than the highly persistent broad spectrum synthetic chemicals. Therefore, use of plant based pesticides is less disastrous and more ecofriendly.