The penetration of the miracidium of Fascioloides magna into the snail host Fossaria bulimoides is followed by examining the process at certain, timed intervals during the course of penetration. The cilia are lost early, but the epithelial cells may detach or they may be carried into the snail body. The apical gland releases vesicles which lose their outer coat in the snail tissue; similarly the vesiculated gland cells release vesicles which lose their coating in the snail tissue where there is a concomitant destruction of snail collagen fibers. Evidence is presented to support the concept of both a mechanical and an enzymatic penetration of snail tissues. Both transmission and scanning electron micrographs are used.