Modifications of tobacco mosaic virus and its RNA caused by treatment with sodium bisulfite were examined. After this treatment, of all the four basic components of naked viral RNA, cytidine residue decreased by about 9.6% and uridine residue increased by about 9.8% but guanosine and adenosine residues underwent no changes at all. On the other hand, no changes occurred to any basic component of RNA extracted from treated virus particles. In a sucrose density gradient made with 0.02M tris-HCl buffer, sedimentation velocity was slower in sodium bisulfite-treated viral RNA than in untreated viral RNA. In a sucrose density gradient made with tris-HCl buffer containing 0.1M NaCl, however, the sedimentation velocity of the treated RNA was the same as that of the untreated RNA. The sodium bisulfite-treated viral RNA decreased rapidly in infectivity. Although the infectivity of the virus particles did not change after sodium bisulfite treatment for 30 minutes, it decreased by about 30% after the treatment for 2 to 48 hours.