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The myxosporean parasite Kudoa thyrsites (Gilchrist) is well recognised as a cause of flesh liquefaction post-mortem in several fishes from both hemispheres. The observation of this condition in cultured mahi mahi, Coryphaena hippurus L., in Western Australia led us to search for possible reservoirs of infection amongst the abundant clupeoid fishes of the area. Prevalent and severe infections were found in Sardinops sagax neopdchardus (Steindachner), which we consider to be a major reservoir host for K. thyrsites in south-west Australian coastal waters. The parasite occurred less frequently in Spratelloides robustus Ogilby. Sardineila lemuru Bleeker and Engraulis australis (Shaw) from the same waters. Infection was not detected in Hyperhphus vittatus (Castelnau), Etrumeus teres (De Kay) or Nematalosa vlatninghi (Munro), which may reflect differences in range or dietary composition. Engraulis japonicus (Temminek & Schlegel) imported from Japan were also infected, but a single sample of E. mordax Girard from California contained only an equipolar Kudoa species. These records expand the known host range of K. thyrsites to 20 species in 10 families, and several of these host species appear to be major reservoirs of infection in certain locations. Flesh spoilage due to the liquefactive effects of the parasite could occur in any of these hosts, and was seen here in S. sagax neopilchardus, S. robustus and E. japonicus in association with mature spores. Detection of infection with ultra-violet light was found to be ineffective because a cellular host response was not mounted by the clupeoids. [ABSTRACT FROM AUTHOR]