Immunohistochemical and electron microscopic examinations were made of a carcinosarcoma of the esophagus in an 80-year-old man. An immunohistochemical examination showed that sarcomatous spindle cells were vimentin-positive, whereas squamous carcinoma cells were keratin-positive. No coexistence of vimentin and keratin in a single tumor cell was found. Electron microscopically, the sarcomatous spindle cells were characterized by well-developed rough endoplasmic reticulum, abundant intermediate filaments, and the occasional presence of peripheral aggregates of microfilaments. No definite desmosomes were identified among these cells. These results appear to indicate that most of the spindle-shaped tumor cells assume fibroblastic cellular features and synthesize the intermediate filament protein usually expressed in mesenchymal cells, even though such tumor cells could be epithelial in origin.