Myxozoans are spore-forming parasites of both freshwater and marine fishes (Lom & Dykova, 1992, Kent et al., 2001; Feist & Longshaw, 2006). The Myxozoa were previously classified as protozoans, although the multicellular state and functional specialization of the cells composing spores were considered to exceed protozoan level (Lom & Dykova, 1992). Indeed, molecular studies demonstrated that myxozoans are metazoans (Smothers et al., 1994, Siddal et al., 1995). However, there were two conflicting views concerning the phylogenetic origin of myxozoans; the Bilateria (Smothers et al., 1994, Schlegel et al., 1996, Anderson et al., 1998, Okamura et al., 2002) vs. the Cnidaria (Siddal et al., 1995). More recently, the Cnidaria-hypothesis has been strongly supported by phylogenetic analyses of protein-coding genes of myxozoans (Jimenez-Guri et al., 2007, Holland et al., 2010). The phylum Myxozoa, of which more than 2100 species in 58 genera are described to date, is divided into two classes, Myxosporea and Malacosporea (Lom & Dykova, 2006). Most of myxozoans are not harmful to host fish, however, some species cause diseases in cultured and wild fish which are problems for aquaculture and fishery industries worldwide. Generally, freshwater myxosporeans appear to be specific at the family or the genus level of the host, while some marine myxosporeans have a low host-specificity. Some examples are mentioned below.