It has been established that the wall of the metacercarial cyst of the strigeid digenean Apatemon (Australapatemon) minor from the leech Erpobdella octoculata shares with Apatemon (Apatemon) gracilis the remarkable ability to expel the metacercaria forcibly from the cyst, an event which presumably occurs when the cysts are eaten by the bird definitive host. When cysts of A. minor are treated in vitro with a solution containing a mixture of bile salts and trypsin following pretreatment with acid pepsin, the metacercaria is expelled in an explosive manner through a canal at the narrow end of the pear-shaped cyst. This expulsion is produced by a sudden and substantial inward expansion of the birefringent wall, as a consequence of which the cyst lumen is virtually eliminated. Expulsion of the metacercaria in this explosive manner also occurs when similarly pretreated cysts are exposed to either bile salts alone or to trypsin, but cysts treated with the latter take longer to respond. When the cyst wall is perforated with a needle, inward expansion of the wall occurs and the metacercaria is forcibly ejected through the perforation. A variety of other pretreatments and treatments was tested but pepsin appears to be the most effective “primer”, apparently producing localized changes which permit the exit of the metacercaria. These observations are discussed in relation to the mechanism of expulsion.