Many plant based products are widely used for their insecticidal and repellent properties for the control of mosquitoes. The present paper reports isolation of mosquito larvicidal bioactive saponin form an indigenous plant found in India –Euphorbia hirta of family –Euforbiaceae. The mosquito larvicidal activity of bioactive saponin isolated from Euphorbia hirta have been tested against Culex quinquefasciatus by exposing IInd and IV instar larvae to four different concentration of the compound. Three trials were performed for each concentration along with control and untreated. 24 hours LC50 and LC90 values were determined using probit analysis method. It was noticed that IVth instar larvae are more susceptible than IInd instar larvae. The results obtained suggest that bioactive compound of Euphorbia hirta could be used in the search for new larvicidal compound of plant origin.