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Generally, malignant sarcomatoid mesothelioma (MSM) has few desquamated tumor cells in the ascitic effusion when compared to those of epithelioid type, and is said to present difficulty in cytopathological diagnosis. There are few reports of the cytopathological features of MSM. We report herein cytopathological features of tumor cells found in ascites of a 58-year-old male patient with MSM. In ascitic fluid, sarcomatoid large atypical polygonal and short spindle cells with irregular nuclei, irregular membranous invagination, and distinct nucleoli were seen. Although there is no definite evidence of mesothelial differentiation in sarcomatoid atypical cells in the ascitic fluid by immunostaining, a diagnosis of peritoneal malignant mesothelioma was made according to the histopathological features of a surgical peritoneal sample and the clinical course of this patient. When various sarcomatoid atypical cells are seen in the ascitic fluid, it seems to be necessary to include MSM in the differential diagnosis.