A laboratory-held summer flounder (Paralichthys dentatus) became moribund and presented gross ulcerative and hemorrhagic lesions, concomitant with a space-occupying lesion in the abdominal cavity and a prolapsed rectum. Edema, hemorrhage, and necrosis of the intestine and edema of the stomach wall were noted upon post-mortem examination. Microscopic examination revealed large numbers of Cryptobia in the submucosa of the gut and in the liver.