We investigated the effects of formulated pesticides on non target organisms in order to evaluate the role played by inert ingredients in the toxicity of agrochemicals. The effects of both technical grade and formulated pesticides on esterase activity of two freshwater snail species Helisoma duryi and Lymnaea natalensis was investigated. Snails from both snail species were exposed to chlorpyrifos, dimethoate, carbaryl, lambda cyhalothrin, deltamethrin or mancozeb for 72 hours. Some of the snails were exposed to technical pesticides while others were exposed to the commercial forms of the pesticides. After the exposure duration post-mitochondrial supernatants of whole snail homogenates were prepared and used to measure choline and non-cholinesterase activity using substrates, acetylthiocholine iodide or α -naphthyl acetate respectively. The results showed that both formulated and technical grade pesticides significantly inhibited esterase activity. The inhibition observed in snails exposed to formulated commercial pesticides was twofold or more when compared to inhibition observed in snails exposed to technical grade pesticides and this was attributed to presence of inert ingredients in formulated pesticides. The enhanced inhibition of esterase activity observed in snails exposed to formulated pesticides indicates that aquatic organisms are potentially at risk to the toxic effects of the unspecified ingredients which are part of formulated pesticides.