The differential diagnosis of malignant soft tissue tumors constitutes a major problem in surgical pathology. A distinction among sarcomas is sometimes difficult, but this is a situation in which histologic examination of representative tissue will give a diagnosis in the majority of cases. Electron microscopy sometimes contributes to the proper diagnosis of sarcomas. For electron microscopy, fresh samples from 110 cases of sarcoma filed at the Department of Pathology, Faculty of Medicine, Kyushu University, were examined. The distinguishing ultrastructural appearance of the following tumors are discussed: Spindle cell sarcomas (leiomyosarcoma 10 cases, malignant Schwannoma 5, fibrosarcoma 5, monophasic synovial sarcoma 7). Round cell sarcomas (rhabdomyosarcoma 10 cases, extraskeletal Ewing's sarcoma 1, malignant neuroepithelioma 1, malignant rhabdoid tumor of soft parts 3, monophasic synovial sarcoma 9). Pleomorphic sarcomas (malignant fibrous histiocytoma 24 cases, pleomorphic liposarcoma 1). Myxoid sarcomas (myxoid malignant fibrous histiocytoma 8 cases, myxoid liposarcoma 8, extraskeletal myxoid chondrosarcoma 4). Sarcomas with a specific pattern. Using electron microscopy, general analysis of cellular morphology and detection of specific structures are helpful for evaluating these sarcomas. Nevertheless, poorly differentiated sarcomas may still pose a diagnostic dilemma, since their specific features are poorly represented.