The planorbid snail Biomphalaria glabrata responded to exposure to either the compatible digenetic trematode Echinostoma paraensei or the incompatible species Echinostoma trivolvis by producing increased amounts of several distinctive plasma polypeptides. These polypeptides characteristically precipitated from plasma when mixed with secreted–excreted products (SEP) of sporocysts or rediae from either digenean species. In contrast, control snails, or snails that had been wounded or infected with bacteria (Serratia marcesens or Staphylococcus epidermidis) showed no obvious plasma alterations and no precipitates formed when their plasma was mixed with SEP. Another planorbid species, Helisoma trivolvis, which displays reverse compatibility for the echinostome species used, also responded to exposure to both echinostomes by increased production of plasma polypeptides that precipitated in the presence of SEP. With some individual variation, these 2 snail species synthesized SEP-reactive plasma polypeptides forming diffuse bands centred at 53, 65, 80–120 and 200 kDa (the latter absent in Helisoma trivolvis). The 53 kDa polypeptides had not been observed before, whereas the others have been noted from B. glabrata. The diffuse 65 kDa band was strongly bound by anti-fibrinogen antibodies, supportive of earlier studies indicating it contains fibrinogen-related domains. The other specified polypeptides were also bound by these antibodies raising the possibility that they too contain fibrinogen domains. The results are suggestive of a general ability of these 2 planorbid snails to detect the presence of echinostomes even if the latter are subsequently incapable of development. The complex response they then mount, one not evoked by other challenges such as wounding or bacterial infection, may represent a dedicated response to a frequently encountered group of pathogenic parasites, the digeneans (echinostomes).