The distribution of the acanthocephalan Pomphorhynchus laevis along the alimentary tract of several species of freshwater fish was studied in both natural and experimental infections. The parasite exhibits a clear preference for a particular region of the alimentary canal, which differs from host species to host species, but it is capable of surviving in all regions of the intestine. In some host species, but not in all, its growth rate may be reduced in regions outside of the preferred site. The parasite remains in the same site throughout the period of infection, emigrations occurring only preparatory to complete loss of the parasite from the host. The presence of cestodes in the region of the preferred site has no effect upon the distribution of the parasite. At high levels of infection the range of the parasite's distribution is extended. The feeding of larval parasites to fish by stomach tube also leads to an extension of range, whereas when fish are allowed to feed on cystacanths removed from their intermediate host the parasite establishes in a more anterior position than usual and in a site that it does not occupy to any extent in natural infections. It is concluded that the distribution of P. laevis in the alimentary canal offish is determined primarily by the process of liberation, activation and establishment of the parasite, and that it normally attaches in the first available space and remains there. Unsuitable physico-chemical or feeding conditions affect only its growth, not the site that it occupies. It is suggested that P. laevis may be atypical in this respect.