Prior exposure of Biomphalaria glabrata to the eggs of an incompatible digenean, Plagiorchis elegans, rendered this snail host less suitable to a compatible species, Schistosoma mansoni. Although P. elegans failed to develop patent infections in B. glabrata, it reduced the production of S. mansoni cercariae by 88%. Concomitantly, host attributes such as reproduction, growth, and survival were compromised. The effect of P. elegans infection was most severe among snails that, in addition, had developed patent schistosome infections. Although few S. mansoni cercariae were produced, egg production by B. glabrata was only 4% of control values. Furthermore, no doubly infected snails survived for more than 3 wk after patency, whereas controls experienced no mortality during the same time period. The above effects were attributable to the establishment and persistence of P. elegans sporocysts in the tissues of the incompatible snail host. Their indirect antagonistic interaction with the larval stages of S. mansoni may be mediated, in part, through their long-term stimulation of the host's internal defense mechanisms. These findings are discussed with a view to use P. elegans and other plagiorchiid digeneans as agents in the biological control of snails and snail-borne diseases.