Attempts to modify satellite tobacco necrosis virus (STNV) chemically to obtain heavy atom derivatives have shown that the virus particle is chemically very inert. Of the many modification methods employed (resulting in formation of covalent bonds), iodination was the only one to provide useful derivatives. A derivative of STNV containing 2.3 iodine atoms per protein subunit was prepared and crystallized. X-ray data were collected on this heavy atom derivative. The X-ray diffraction pattern from the derivative crystals showed significant intensity differences compared to that of the native crystals. Modified peptides of iodinated STNV were isolated and the sites of modification were determined. Of the 2.3 iodine atoms found to react with STNV on the average, 1.5 atoms were bound to one of the four histidine residues and the residual 0.8 iodine atoms were observed bound to at least two of the four tyrosine residues.