One of 373 oysters examined as part of a histological survey of oysters from Apalachicola Bay, Florida, USA, had a concurrent blood cell proliferative disorder and a protistan infection. The neoplastic blood cells (leukocytes) were found throughout the vesicular connective tissues and blood spaces in sections of the oyster. These proliferating cells resembled the neoplastic cells described by Couch (1969) and Farley (1969) from other specimens of Crassostrea virginica from Chesapeake Bay, Maryland, USA. Mitotic figures were abundant in foci of the neoplastic tissue. Epithelial tissues of the gut of this oyster were infected by spore and schizogonic stages of Dermocystidium marinum, a presumed pathogenic protist of oysters. Spores of this protist and other proliferative stages were observed in connective tissues and blood spaces. There was no morphological evidence that the neoplastic blood cells were related to the protist life cycle stages.