Cases of pleural mesothelioma induced by five species of chrysotile and three species of crocidolite from China in rats are compared. The rate of induction in the chrysotile group (43.1%) was slightly lower than that in the crocidolite group (56.7%). The survival time of the first case of mesothelioma in the chrysotile group was 408 days and in the crocidolite group 377 days. The major histological types of mesothelioma induced by chrysotile were epithelial and mixed, but the major type in the crocidolite group was fibrous. The extent of differentiation in all mesotheliomas, mainly intermediate and low, was nearly the same.