1. Electron microscopic observations were made on DNA fibers in the mitochondria of tumor cells induced by various kinds of DNA viruses, RNA viruses, and chemical carcinogens, and also in those of normal and regenerating rat livers and cultured liver cells. Intramito-chondrial DNA fibers were observed most frequently in the sectioned specimens of adenovirus type 12-induced or SV 40-induced hamster tumor cells, but were hardly observed in normal and regenerating liver cells and other tumor cells. 2. The DNA fibers disappeared by treatment with DNase, but not with RNase. 3. The DNA fibers in adenovirus-induced tumor cells were easily isolated by osmotic shock and observed as circular DNA molecules by rotary shadowing with the electron microscope, while DNA molecules were hardly isolated from rat liver mitochondria by the same treatment. 4. On the ultracentrifugal fractionation of sonically disrupted adenovirus-induced tumor mitochondria, DNA were found to be contained mostly in the supernatant fraction by chemical analysis and by electron microscopic observation. 5. DNA fibers observed in the mitochondrial matrix in the sectioned specimens were proved to be identical with the isolated circular DNA molecules. 6. On the ultracentrifugal fractionation of sonically disrupted rat liver mitochondria, DNA were found to be contained mostly in the membrane fractions by chemical analysis. However, DNA molecules were difficult to observe with the electron microscope in the sectioned specimens of the rat liver mitochondrial membrane fractions. Nevertheless, they were isolated by phenol extraction from the membrane fractions, and were observed as circular DNA molecules with the electron microscope. 7. Appearance of the DNA fibers in the mitochondrial matrix seems to be associated with the division cycle of mitochondria, and in the mitochondria where DNA fibers are not observed in the sectioned specimens, the DNA molecules are supposed to be hidden by firmly attaching to the inner membrane, and to be isolated by chemical extraction.