Seven cases of synovial sarcoma were studied by electron microscopy. The 4 cases of the monophasic spindle type contained elongated cells arranged in sheets and cords; ultrastructurally the cells contained many cytoplasmic processes and were scattered in an amorphous matrix. The monophasic spindle type demonstrates ultrastructural characteristics that are unique and serve to differentiate it from the biphasic pattern and from fibrosarcoma. Three cases of the biphasic variety demonstrated cytologic characteristics of a true glandular epithelium and a malignant spindle cell stroma. The epithelial nature of the biphasic variety is a feature that raises doubt whether synovium is the stem cell.