Assessing the cumulative toxicity of pesticides in mixtures has been an enduring challenge for environmental health research as well as ecotoxicology for the past several decades. Usage of single pesticides in agriculture belonging to one group may not be possible in nature, Group of those belonging to one class or more is after the real picture in the environment. Hence, it is not going to reveal a clear picture of assessment of toxicity singly but perhaps the mixed composition may be correct. Hence a study was under taken to evaluate the toxicity of the three different groups of organophosphate pesticides viz quinalphos [Organothiophosphate group - phosphorothionate], malathion [Dithiophosphate group - phosphorothiolothionate] and monocrotophos [Orthophosphate group] in mixed ratios (1:1:1, 2:1:1, 1:2:1 and 1:1:2) of commercial grade formulations to find out the combined interactions of the organophosphorus pesticides on the fish Channa punctatus. The nature of interactions in the mixture of the toxicants were evaluated by linear ‘S’ Index method and Mixture Toxicity Index method was applied to verify the nature of the combined interactions of the toxicants. More than additive nature of the combined effect observed in the mixture experiment demonstrated that all components contribute to the overall effect of the mixture.