An ultrastructural study was conducted on a total of 24 tumors selected from 70 tumors Induced by intramuscular injection of Ni3S2in Fischer and Hooded rats. Well differentiated rhabdomyosarcomas were predominantly composed of strap‐like cells containing numerous myofilaments occasionally forming S‐line‐like structures, whereas poorly differentiated rhabdomyosarcomas were made up mainly of round or oval cells with abundant cytoplasm. Embryonal rhabdomyosarcomas contained cells resembling the mast cells in myxomatous areas as well as spindle‐shaped myoblast‐like cells. Mesenchymal tumors exhibited morphological characteristics of the lymphoid cells. The tumor cells from leiomyosarcoma‐like tumors showed myoblast‐like appearance. Tumors diagnosed as fibrosarcoma‐like tumors at the light microscopic level demonstrated the ultrastructural characteristics of fibrosarcomas. The possibility of a multicentric origin of the tumor cells in Ni3S2‐induced tumors h