Different fish-parasitic myxosporeans are characterized by a varying degree of host specificity. While certain species have very strict specificity and can colonize only a single fish species, most species of Myxosporea share several closely related fish species as hosts. Myxosporeans are tissue-specific parasites and always develop in a specific host tissue. The author assumes that the development of all myxosporean species includes an intracellular stage. Organ specificity depends on the tissue types that occur in the given organ. The author distinguishes three main types of development: Myxobolus-type, Hoferellus-type and Sphaerospora-iypt development. Myxobolus-type myxosporeans develop in large Plasmodia (or, according to an incorrect but generally used term: cysts).