Six solitary (localized) pleural mesotheliomas were studied by light and electron microscopy. All the lesions were benign and were composed mainly of fibrous tissue of variable cellularity with or without cystic spaces lined by round cells. The lining cells of the cysts and the adjoining round plump cells were interpreted as true neoplastic cells of the fibroblast type. Results of light- and electron-microscopic study of human mesothelial cells and fetal mesothelial cells of rats were compared. The cytoplasmic organelles of the tumor cells were generally scanty, though rough endoplasmic reticulum, sparse mitochondria, intracellular bundles of fibrils, and numerous polysomes were seen. Some tumor cells had junctional apparatus and basement membrane and showed interdigitation of the plasma membrane. These cells lined the cystic spaces irregularly and also proliferated into the surrounding fibrous tissue, where they assumed a spindle shape and resembled fibroblasts. Ultrastructurally, the tumor cells were similar to mesothelial and stromal cells of fetal rat pleura. We speculated that the solitary (localized) mesotheliomas were probably derived from coelomic epithelium and that tumor cells remained undifferentiated or revealed minimal differentiation toward mesothelial cells.