Five benign histiocytomas of varying pattern and three malignant fibrous histiocytomas have been studied ultrastructurally. An essentially biphasic pattern of histiocytes and fibroblasts was present. In addition fibrohistiocytes, myofibroblasts and undifferentiated cells were present in some of the tumours. The histogenesis of this group of tumours is discussed. The value of electron microscopy in establishing the diagnosis in difficult cases is emphasized, but it is not considered useful in distinguishing benign from malignant cases.