Cysts of Myxobolus osburni occurred throughout the pancreas in 119 of 150 (79.3%) pumpkinseed (Lepomis gibbosus) from West Lake Okoboji, northwest Iowa during the summer, 1980. None of 341 bluegill (Lepomis macrochirus) were infected. Lobate cysts (up to 3 mm diameters) adjacent to the gall bladder contained sporulating plasmodia and mature spore masses. Inflammatory responses in the infected pancreas included engorged blood vessels, endocrine cell nuclei pyknosis, leukocyte (mostly lymphocyte) infiltration, fibroblast proliferation, dark pigment deposition and fibrosis. Host reaction was most pronounced in pancreas containing unencapsulated spore aggregates. Spore variability and plasmodial structure are also described.