The habitat specificity of the acanthocephalan Acanthocephalus clavula in the alimentary tract of eels was studied in natural infections and in fish maintained experimentally under different regimes. The parasite exhibited a preference for a particular region (55 to 65%) of the alimentary canal which did not differ in different species of host, but habitat specificity was not very precise as there was considerable variation between individual hosts and A. calvula was capable of surviving and maturing in all regions of the intestine. The mean position of males was slightly anterior to that of females, and the over-all sex ratio was in favour of females. At high levels of infection the range of the parasites was extended and the means position was significantly more anterior when compared to single worm infections. The parasite remains in the same site throughout the course of an infection: there was no posterior migration and gravid females occurred in the same proportion in all regions of the intestine. The mean position of the parasite did not change when the host was starved, or maintained in 100% sea water or with reduced aeration or at high temperature. The results were compared with the habitat specificity exhibited by other species of freshwater fish acanthocephalans, and it was concluded that A. clavula was as adaptable and hardy as its present host, the eel.