Abstract Tipula iridescent virus is shown to contain 12.4% deoxyribonucleic acid (DNA) and 5.2% lipid, most or all of which is phospholipid. No ribonucleic acid (RNA), neutral polysaccharide, or amino polysaccharide can be detected, however, and indirect evidence indicates the absence of acid-soluble phosphorus compounds. The remainder of the virus material appears to consist mostly, or perhaps entirely, of protein, including some phosphoprotein. The DNA of the virus contains only the usual four bases, with the molar amount of adenine (A) equal to thymine (T) and of guanine (G) equal to cytosine (C). The molar dissymmetry ratio (A + T) (G + C) = 2.20 . The particle weight is 1.22 × 109, and the diameter of the frozen-dried particle is 130 mμ. The two figures are consistent with one another upon assuming a particle density of 1.39, which can be estimated from the chemical composition of the virus. The results are discussed with reference to other insect viruses and viruses in general.