Organophosphate pesticides (OPs) have been intensively used as insecticides in agriculture; after entering the aquatic environment, they may affect a wide range of organisms. A conductometric enzymatic biosensor based on lipase extracted from Candida rugosa (CRL) has therefore been developed for the direct and rapid quantitative detection of organophosphate pesticides: diazinon, methyl parathion and methyl paraoxon in water. The biosensor signal and response time were obtained under optimum conditions, the enzyme being immobilised in the presence of gold nanoparticles. Under these conditions, the enzymatic biosensor was able to measure concentrations as low as 60 mu g/L of diazinon, 26 mu g/L of methyl parathion and 25 mu g/L of methyl paraoxon very rapidly (response time: 3min). Moreover, this CRL biosensor was not sensitive to interferences such as carbamates. It presented good storage stability for 21days when kept at 4 degrees C and it was successfully applied to real samples.