Tumor tissues from six patients with alveolar soft part sarcoma (ASPS) were investigated ultrastructurally to determine the presence of smooth tubules associated with plasmalemmal invaginations. Two different types of smooth tubular structures were identified: an aggregated and a dispersed type. The aggregated type (found in one of the six ASPSs) showed marked aggregation of many elongated smooth tubules without ribosomes associated with the plasmalemma. In the dispersed type (observed in the remaining five ASPSs), there was a conspicious appearance of a few smooth tubules scattered throughout the cytoplasm. Smooth tubules were the main component in both types, suggesting that both types appeared to have the same origin, although there were some differences in number, appearance, and distribution of the tubules between the two types. The smooth tubules in both types were classified into three different subtypes on the basis of materials in the tubular lumen. Ultrastructual observation and a plasmalemmal tracer-method, showed smooth tubules in continuity with the plasmalemma in three of the six cases, indicating that the tubules originated from the plasmalemma. Since the tubular structures were found in the tumor tissues of all six patients, they are probably one of the characteristic features of ASPS.