Several 6-carboethoxy-(2-substituted)-1,3,4-thia/oxadiazolo[2,3-b] pyrimidin-5-ones 3 and 6 and 5-amino-(2-substituted)-1,3,4-thia/oxadiazolo[2,3- b]pyrimidin-7-ones 4 and 7 have been synthesised from the synthon 2-amino-(5-substituted)-1,3,4-thia/oxadiazoles 1 and diethyl ethoxymethylenemalonate and ethyl cyanoacetate respectively. The molluscicidal activity of few title compounds have been evaluated against snail Lymnaea accuminata.