Various physical and chemical factors were studied to determine their effects on the viability of encysted metacercariae of Echinostoma caproni. Viability was equated with chemical excystation in an alkaline trypsin–bile salts (TB) medium. Control cysts showed excystation percentages of >90% in TB. Excystation proved to be a more reliable criterion of cyst viability than observations by light microscopy. Isolated cysts and cysts left in the snail (in situ cysts) were studied. Generally, in situ cysts proved more resistant to various physical and chemical treatments than did isolated cysts. Cysts stored for 7 days at 28 C in a Locke's 1:1 solution showed 97% excystation, suggesting that cysts of this species would survive postal delays during shipment. Of numerous marinades tested, the one that was most harmful to isolated and in situ cysts was vinegar. Isolated and in situ cysts were killed by boiling (100 C) for 1 or 3 min, but freezing at −10 C did not kill all isolated or in situ cysts after 24 hr. Concentrations of potassium permanganate ranging from 300 to 1,200 mg/L killed most isolated cysts within 5 min, but in situ cysts survived these concentrations for 24 hr. Concentrated solutions of NaCl and sucrose had no effect on the viability of isolated and in situ cysts, suggesting that their use in food preparations for molluscs would not be effective in killing echinostomatid cysts in tainted snail tissues.