Electron microscopic study of a granular cell tumor of the urinary bladder revealed myofilaments in the tumor cells, documenting the myogenous derivation of this neoplasm. This is in contrast to the ultrastructural features of the common variety of granular cell tumors, currently interpreted as being indicative of a schwannian histogenesis. The granules of these tumors are similar in ultrastructural appearance regardless of the location of the neoplasms, and are reminiscent of lysosomes and cytosegresomes. It is suggested that granular cell tumors may derive from different cell types.