The albino strain of Biomphalaria glabrata is capable of developing an acquired resistance to echinostomes that is relatively specific, i.e., strong against Echinostoma lindoense, challenged by the homologous species, moderate or weak against closely related E. paraensei and E. liei, and nonexistent against Paryphostomum segregatum and Schistosoma mansoni. The present results show that acquired resistance induced by E. paraensei is much stronger against the heterologous E. lindoense than against the homologous species. These observations indicate that although the response of snails with acquired resistance to a challenge exposure is relatively specific, the stimuli for inducing that resistance may be nonspecific.