A case of malignant epithelial mesothelioma of the peritoneum diagnosed by fine needle aspiration cytology is described. The smear showed many Individually scattered or clustered large round malignant epithelial cells intermingled with relatively small nonneoplastic mesothelial and mesenchymal cells. Papillary configurations with thick fibrous core were also seen. The malignant cells were virtually reminiscent of reactive mesothelial cells but they were larger in size and had more prominent nucleoli and more frequent binucleated or multinucleated cell formations than reactive mesothelial cells. The characteristic features of malignant cell of mesothelioma compared with the metastatic adenocarcinoma were relatively uniform cellular size, prominent round nucleoli, large round vesicular nuclei with finely granular chromatin pattern, smooth nuclear membrane, abundant glassy cytoplasm rather than bubbly mucin-containing cytoplasm and fuzzy cell border.