Although the three-spined stickleback Gasterosteus aculeatus L, is a common euryhaline fish, the salinity tolerances of its parasites have not been studied. The present survey was undertaken to remedy this. Sticklebacks were caught at twenty-five freshwater, brackish and marine sites in Southern England, Scotland and the Isle of Man, and yielded thirty-two species of parasite. As a result-of this field work, the effect of the following parameters - geographical variation, seasonal variation, sex of the host, speciation of the host, parasite interdependence, length of the fish host and salinity - on the parasite, fauna were analysed. The salinity tolerance of a number of ectoparasites was studied in detail, and the sodium ionic content of intestinal fluid of starved and fed sticklebacks in various salinities were measured. As a result of all these studies, the parasites were assigned to six salinity-range categories and a comprehensive fauna list representing freshwater, brackish water and marine water was built up.