Site selection, and the means to access specific sites, is a keystone of parasitology. I evaluated migration and site selection behaviours of metacercariae of two congeneric species of strigeoid trematode throughout growth and encystment phases in the fathead minnow. Results showed that pre-encystment stages of Ornithodiplostomum ptychocheilus migrate along specific neural tracts to access sites in the optic lobes of the brain. Conversely, pre-encystment stages of Ornithodiplostomum sp. migrate via direct penetration, or via the vascular system to access visceral organs, especially the liver. Remarkably, both species have a bi-phasic pattern of development, with growth and encystment occurring in unique sites. Finally, I examined patterns of rodlet cell proliferation and maturation in response to growth and encystment phases of O. ptychocheilus. Cell densities were low (<11/mm2) in brain tissue adjacent to 1 and 2 week old metacercaria, but peaked to approximately 210/mm2 at 6 weeks. These results shed new light on the potential function of these enigmatic cells. / x, 125 leaves : ill. ; 29 cm.