Seven malignant fibrous histiocytomas containing myxoid areas were studied to clarify the nature of the cells constituting the myxoid area with emphasis on multivacuolated cells known to be present in myxoid areas. Ultrastructurally, myxoid areas were, like conventional cellular areas, composed mainly of atypical tumor cells, small histiocytic cells, and occasional lymphoid cells. The tumor cells had large, indented nuclei and well-developed organelles. They showed, despite some variations in subcellular structures, strong resemblance to fibroblast and could be attributable to one cell type. The large vacuoles in multivacuolated tumor cells seemed to be formed by fusion of small vacuoles that appeared in the cytoplasm and later came to communicate with the extracellular space. The small histiocytic cells, cytologically bland and, occasionally, being in contact with lymphoid cells, were readily distinguishable from the tumor cells and were considered to be infiltrating reactive cells.