A young, male, Pacific White-Sided Dolphin was found disoriented and circling. During its twenty-one months of captivity, the animal responded poorly and appeared unable to identify its trainer. Following a short period of reduced appetite but few other clinical signs, the animal was found listing to one side and subsequently died. Thorough post-mortem examination showed few body fat stores and evidence of septicemia due to Edwardsiella tarda. Microscopic examination of the brain revealed numerous parasitic ova in the right medial frontal area of the cerebrum and in the left posterior cingulate gyrus. Large, irregular areas of polioencephalomalacia were associated with the ova and adjacent to areas in which ova were present. The morphologic appearance of these ova and their location were strongly suggestive of Nasitrema sp. It is speculated that these chronic lesions may have been responsible for the initial disorientation, learning disabilities and predisposition to bacterial infection.