Two compounds, a pyrethroid viz., decis and a carbamate sevin both effective molluscicides against the snail Lymnaea acuminata demonstrate synergistic activity when mixed in a 1:46 ratio (decis:sevin). The observations show that the LC values decreases sharply. For example, the 96h LC 1 0 , LC 5 0 and LC 9 0 of decis decreases 272.72, 112.82 and 10.92 times respectively. When the esterase inhibitor MGK-264 is mixed with the binary mixture in a 1:46:5 ratio (decis:sevin:MGK) it was found that the lethality of the tertiary mixture was much higher than the binary mixture of decis and sevin. It seems that sevin and MGK-264, both being esterase inhibitors prevent the deactivation of the decis in the body of the snail Lymnaea acuminata and thereby increase the toxicity of the latter.