Molluscicides are useful in schistosomiasis control. Laboratory surveys proved that both Pittosporum tobira (Pittosporaceae) and Tribulus macropterus (Zygophyllaceae) have marked molluscicidal activity. The recorded LC₉₀ values showed that P. tobira was more active than T. macropterus as the LC₉₀ were 105 and 160 ppm for the two plants respectively against Biomphalaria alexandrina the natural intermediate host of Schistosoma mansoni in Egypt. Also both fruits and leaves of T. macropterus showed a noticeable effect. As many chemicals may be mixed in the flowing water with plant molluscicides, so it was recommended to further study the combined effect of the two tested plants and some chemicals on adult non- infected B. alexandrina snails. Copper sulphate was the only chemical that showed a high synergistic effect with both plants during pre-and simultaneous exposure while the effect of most other chemicals was additive or antagonistic. A synergist can potentiate the efficacy and reduce the dosage of molluscicides. The study proved that the plants also showed a molluscicidal action on five other pulmonate species, although they are not all medically important snails. A 90% of snail mortality was obtained for all species at 97-220 ppm of P. tobira and 120-270 ppm of T. macropterus during 24 hours of exposure.