Experiments on x-ray-induced mutations of the tobacco mosaic virus (Gowen) are analyzed on the basis of the ionization theory. The size of the volume within which the primary process of mutation develops is calculated on the basis of three alternative assumptions; viz., (1) that this volume consists of protein, (2) that it is nucleic acid, (3) that it is phosphorus. It is found that the volume calculated under assumption (1) is identical with the hexagonal cell unit which Bernal and Fankuchen found in the virus protein by x-ray diffraction. Assumptions 2 and 3 lead respectively to the conclusions that one-fourth of the total nucleic acid content or one-half of the total phosphorus content is involved in the mutational process. The relation between the induced and the spontaneous mutation is examined and it is found that natural ionizing radiations are completely insignificant for the spontaneous mutation of the virus.