The histologic and ultrastructural morphology of three cases of Ewing's sarcoma of soft tissue are described and the fine structural features of extraskeletal Ewing's sarcoma are compared to those of similar round-cell tumors that are considered in the differential diagnosis. By light microscopy, these tumors are indistinguishable from Ewing's sarcoma of bone. Ultrastructurally, the salient features are also comparable to Ewing's sarcoma of bone and include: 1) absence of surface modifications; 2) cell-contact sites in the form of small thickenings of apposed membranes and large desmosome-like specializations; 3) undifferentiated cytoplasm usually containing abundant glycogen and occasionally nonspecific microfilaments; 4) significant variation in shape and irregularity of nuclear profiles. The ultrastructural features of extraskeletal Ewing's sarcoma are sufficiently distinctive to allow separation from from other small-cell malignant neoplasms in the majority of cases.