The ultrastructural features of one embryonal and two pleomorphic rhabdomyosarcomas in adults are described. The presence of thick myofilaments in formations suggesting sarcomeres and a rare arrangement of mitochondria in tandem allow for identification of the spindle cells in embryonal rhabdomyosarcoma as skeletal muscle cells. Aggregates of similar filaments could be detected in cells comprising alveolated areas and those of the pleomorphic rhabdomyosarcomas, although the sarcomere-like arrangement was less conspicuous. Cells of both histologic types contained cytoplasmic rods of electron-dense material and abundant, dilated, coarse, endoplasmic reticulum. The former resembled the bodies found in nemaline myopathy but lacked the internal periodic bands of such structures. Changes in endoplasmic reticulum were most pronounced in cells comprising the pleomorphic rhabdomyosarcomas, and frequently the dilated cisterns contained homogeneous or vacuolated electrondense material. Intranuclear inclusions frequently observed in cells of pleomorphic rhabdomyosarcoma were demonstrated to be portions of entrapped cytoplasm. Electron microscopic examination can support a diagnosis of rhabdomyosarcoma in certain cases of malignant mesenchymal neoplasms which have recognizable diagnostic features except that cytoplasmic striations are not seen. Contrary to the impression given by light microscopy, spindle cells comprising embryonal rhabdomyosarcoma appear better differentiated than those of alveolar or pleomorphic forms.