X cell lesions (papillomas and pseudobranch tumors) have been reported in the literature to involve fishes of 3 Orders (Pleuronectiformes, Gadiformes, and Perciformes) covering about 40 species. For some 70 years these lesions have been considered neoplastic or pre-neoplastic. In imprint preparations in this study, X cells stained by Wright's, Feulgen, and a DNA-binding fluorescent dye (bisdenzimidazole), the features of the mitotic cycle of X cells were visualized in detail and found to be closely similar to those of hartmannellid amoebae. Mitoses of X cells differed markedly from mitoses of host fish cells. In multinucleated plasmodia-like forms, mitoses of the several nuclei in a given cell were synchronized. Isozyme studies revealed starch gel electrophoresis bands extractable from X cells but not from host fish cells, active in at least 4 to 6 enzyme systems. Flow fluorometric quantitation of DNA showed the X cell population to be sharply distinct from that of host fish cells, with X cell/fish cell DNA ratio of about 1:3. It was concluded that X cells are parasitic rather than neoplastic in nature.