The efficacy of leeches, as biological agents, in control of snail intermediate hosts of schistosomiasis (Bulinus truncatus, Biomphalaria alexandrina) and fascioliasis (Lymnaea natalensis) as well as their effect on the non-target snails Physa acuta, Melanioides tuberculata and Cleopatra bulimoides was eva luated. Two glossiphoniid snail leeches, Helobdella nilae and Alboglossiphonia con jugata were used. They destroyed egg masses and young snails more rapidly than adult ones. H. nilae showed a stronger destructive effect than A. conjuga ta. In a descending order, it preferred L. natalensis followed by B. truncatus, B. alexandrina, Ph. acuta, M. tuberculata and lastly C. bulimoides. But, A. conju gala preferred L. natalensis followed by B. truncatus, Ph. acuta, M. tubercula Ia, B. alexandrina and lastly C. bulimoides. The detailed diagnostic morpholo gy and biology of the two leeches were given. Keywords: leeches, Helobdella nilae and Alboglossiphonia conjugat, Lymnaea natalensis, Bulinus truncatus, Biomphalaria alexandrina, Physa acuta, Melanioides tube rculata and Cleopatra bulimoides.