To determine the relationship between the species richness, diversity of helminth communities, and migration distance during upward migration from coast to freshwater, helminth communities in the anadromous fish Coilia nasus were investigated along the coast of the East China Sea, the Yangtze Estuary, and 3 localities on the Yangtze River. Six helminth species were found in 224 C. nasus. Changes in salinity usually reduced the survival time of parasites, and thus the number of helminth species and their abundance. Except for the 2 dominant helminths, the acanthocephalan Acanthosentis cheni and the nematode Contracaecum sp., mean abundance of other 4 species of helminths was rather low (,1.0) during the upward migration in the Yangtze River. Mean abundance of the 2 dominant helminths peaked in the Yangtze Estuary and showed no obvious decrease among the 3 localities on the Yangtze River. Mean species richness, Brillouin's index, and Shannon index were also highest in the estuary (1.93 ± 0.88, 0.28 ± 0.25, and 0.37 ± 0.34, respectively) and did not exhibit marked decline at the 3 localities on the Yangtze River. A significant negative correlation was not seen between the similarity and the geographical distance (R 5 20.5104, P 5 0.1317). The strong salinity tolerance of intestinal helminths, relatively brief stay in the Yangtze River, and large amount of feeding on small fish and shrimp when commencing spawning migration perhaps were responsible for the results.The anadromous Japanese grenadier anchovy, Coilia nasus (Temminck & Schlegel, 1846), is distributed in the northwestern Pacific. The fish usually feeds and overwinters in the coastal waters and spawns in freshwater. When the anchovy population distributed in coasts of the East China Sea commences migration, the fish can be first found in the Yangtze Estuary in February and the middle reaches of the Yangtze River in April (Yuan et al., 1980;Yuan, 1987;Li et al., 2007). The anchovy returns to the coasts when spawning is completed from April to October in the same year in the Yangtze River (Yuan et al., 1980;Yuan, 1987;Li et al., 2007).With adaptation of fish to a decline of salinity, parasites in the fish also need to cope with changes in the osmotic pressure. A long stay would reduce survival of the freshwater parasites in salt water or marine parasites in freshwater. The mean intensity of 1 freshwater ectoparasite and 2 endoparasites in Oncorhynchus nerka decreased when reared in seawater for 32-40 wk (Bailey et al., 1989). Survival time of the nematode larva Anguillicoloides crassus in intermediate and paratenic hosts was maximal in freshwater and declined with an increase in salinity (Kirk et al., 2000). Populations of the freshwater monogenean Gyrodactylus salaris in Atlantic salmon Salmo salar declined and became extinct when salinity increased from 7.5% to 33.0% after a few minutes (Soleng and Bakke, 1997). Although some Anisakis spp. larvae with high salinity tolerance survived for more than half a year in freshwater, some survived less than...