The Microsporida are obligate, intracellular parasites which occur in the epithelial, connective, muscular, nervous, and germ tissues of marine, euryhaline and freshwater fishes. Aspects of life stages, transmission, effects on host, host and tissue specificity, and zoogeography of the Microsporida in fishes are reviewed. Biology of Plistophora ovariae is summarized and new findings on peroral transmission of Plistophora ovariae are given.