Various physico-chemical factors associated with the in vitro excystment of metacercariae of Zygocotyle lunata were studied. Metacercariae could excyst with or without the presence of trypsin or bile salts, although the rate of excystation was slower in the absence of both. Crude ox-bile extract promoted faster excystment than did more refined sodium taurocholate. Pretreatment of cysts with pepsin also resulted in faster excystment rates. Optimal excystation occurred at 40 degrees C and between pH 5.5 and 7.5. The broad pH and temperature ranges and the non-specific enzyme/bile salt requirements for excystation were correlated with the wide range of vertebrate hosts used by this trematode.