Various systems of locomotion have received much study in past years (Gray, 1928; Prosser, et al., 1952; Weber, 1958) and continue to be of great interest. The growth of electron microscopy and associated techniques has facilitated the study of the ultrastructure of locomotor organelles and tissues (Fawcett and Porter, 1954; Roth, 1956; Huxley, 1957; et al.). Although a complete understanding of the numerous mechanisms of locomotion is still lacking, recent studies utilizing the electron microscope have yielded much information concerning the means by which animals and plants move. The rapid, whip-like movements of the tail of Himasthla quissetensis cercariae aroused the authors' interest in the type of locomotor system responsible for these movements. A review of the pertinent literature revealed that Miller and Northup (1926) had described many details of this cercaria but did not describe the tail structure. Stunkard (1938) described in more detail the cercarial structures but gave only the dimensions of the tail and the location of the excretory pores. Numerous descriptions of the tail structure of other cercariae have been given (Vickers, 1940; Dawes, 1956; Pearson, 1956; et al.) and these reveal that the cercarial tail contains both striated and smooth muscle.