In the search for new plant molluscicides, it was found that the dry powder of two diterpene -containing plants; Euphorbia peplus and seeds of Azadirachta indica exhibited high molluscicidal activity when tested against Biomphalaria alexandrina and Lymnaea cailliaudi snails; the Egyptian intermediate hosts of Schistosoma mansoni and Fasciola species. The recorded LC₉₀ values of E. peplus were 50 ppm and 47 ppm against the two snails respectively, while that of A. indica were 100 ppm and 91 ppm. Different ages of B. alexandrina snails showed different susceptibility towards the action of both plants, with the newly hatched snails being the most sensitive age. Increase in temperature within the optimal range and increasing time of exposure improved the potency of the plants, while presence of mud and alkaline media reduced it. Storing the molluscicidal solutions of the plants for one week greatly reduced the plants toxicities against the snails. However, storing they dry powder of the plants for a whole year did not affect their potencies. Also, the effect of repeated application of LC0 of the plants on the susceptibility of B. alexandrina was investigated. The absence of ovicidal activity of the plants against B. alexandrina eggs was discussed.