Two species of coelozoic myxozoans were examined using scanning and transmission electron microscopy. The ultrastructure of sporogenesis of Chloromyxum trijugum and pathology associated with Chloromyxum trijugum infections of gallbaldder tissue in bluegill (Lepomis macrochirus), white crappie (Pomoxis annularis), black crappie (Pomoxis nigromaculatus), and pumpkinseed (Lepomis gibbosus) were described. The ultrastructure of the myxosporean species, Myxobilatus noturi, was also described from the urinary bladder of tadpole madtom (Noturus gyrinus). Fish samples were collected from artificial impoundments in Story County, Iowa, small streams in Northern Iowa, or from Pool 7 of the Upper Mississippi River (Wisconsin);The events of sporogenesis in C. trijugum were similar to those reported for other myxozoans. The majority of plasmodia were attached to the mucosa, but free floating plasmodia were also observed. Only slight differences in plasmodia infecting bluegill, crappie, or pumpkinseed were apparent. Mitochondria, endoplasmic reticulum, and golgi were more prevalent in the plasmodia associated with bluegill and pumpkinseed than with crappie;Pathology was limited to the apical regions of the infected host cell. Severe cell damage was more prevalent in L. gibbosus and Pomoxis spp. than in L. macrochirus. Progressive degeneration of epithelial tissue was related to the interdigitation of host cell tissue to plasmodial extensions;Free and tissue associated plasmodia of Myxobilatus noturi were observed. The plasmodia were polysporous and limited by a single membrane. Mitochondria within the plasmodial endoplasm contained circular electron dense bodies. Two distinct plasmodial forms were observed with scanning electron microscopy. The smaller (5.6 [mu]m x 5.0 [mu]m) round shaped plasmodia were located in the host cell crevices. The larger (13.2 [mu]m x 4.5 [mu]m) ellipsoidal plasmodia rested on the apical surface of the mocusa. Sporogenesis was not initiated by envelopment of generative cells in M. noturi. However, other sporogenic events were typical of those reported for other myxozoans. Myxobilatus noturi, like C. trijugum, formed pseudopodia, tight-like junctions with host tissues, and extended pseudopods in between adjacent host cells. Because urinary and gall bladder mucosa is replaced rapidly, unless the pseudopodia reach areas below the mucosa, pathosis is minimal.