The structure of the body-wall muscle of the ascidian, Halocynthia roretzi, was investigated by electron and light microscopy. Thin (about 8 nm in diameter), (18–20 nm in diameter), and 10-nm filaments were seen in this muscle cell. These filaments, however, did not have the regular arrangement of a striated or oblique structure. We also found by three-dimensional reconstruction that this muscle cell is multinucleated. Based on these observations, we report that the body-wall muscle of the ascidian, Halocynthia roretzi, is smooth and multinucleated.