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Adults of the African ampullariid snail Pila ovata were examined for their ability to control laboratory populations of the pulmonate snail Biomphalaria pfeifferi, a widespread, intermediate host of the human pathogen Schistosoma mansoni in sub-Saharan Africa. In a 6-week experiment conducted in large (100 x 60 x 60 cm) outdoor tanks containing floating macrophytes (Nymphaea caerula) and initially set up with one adult ampullariid for every three adult pulmonates, the numbers of B. pfeifferi egg masses were always about half those in similar tanks without P. ovata. Although, by week 6, the numbers of B. pfeifferi in the control tanks (without ampullariids) had increased 5-fold, from an initial mean of 30 snails/tank, there was no significant increase in the numbers of B. pfeifferi in the experimental tanks (containing ampullariids). Results of experiments conducted in indoor glass aquaria indicated that adult P. ovata rapidly attacked egg masses or neonates (< 2.5 mm shell diameter) of B. pfeifferi but had no effect on the adults. The adult ampullariids also significantly decreased cover by floating macrophytes over a 6-week period compared with that in similar but ampullariid-free aquaria. This decrease in plant cover is relevant to biological control of the schistosome vectors as macrophytes serve as food, shelter and oviposition sites for pulmonate snails. The present result indicate the ability of P. ovata to inhibit multiplication of B. pfeifferi populations, at least under laboratory conditions, both directly, through predation, and indirectly, by competition for resources. Pila ovata may therefore prove useful in the biological control of medically important, pulmonate snails.