The study isolated nine diterpenes from Euphorbia paralias and tested their biological activities. Molluscicidal activity was evaluated against Biomphalaria alexandrina using the World Health Organization procedure, and anti-feedant activity was assessed against third-instar larvae of Spodoptera littoralis via a conventional leaf disk method. Compounds 5 and 6 were observed to be good insect antifeedants for the first time. In addition, compounds 5, 8 and 9 had high molluscicidal activities against Biomphalaria alexandrina, particularly the paraliane diterpene 9. These findings suggest that aquatic snail control using these compounds might be feasible and further work on this is recommended.