IN addition to the ten double stranded segments of genome RNA, reovirions contain many hundreds of single-stranded oligoribonucleotides from 3 to 20 residues long, which fall into two classes. First, those precipitated by acid or ethanol and very rich in adenine^1–2; second, those poorly or not precipitable and therefore shorter, such as the trinucleotidepppGpCpU_OH^3,4. The former class of molecules comprises the fraction previously designated “A-rich RNA”¹, which has been shown to be a component of all three serotypes of human reoviruses², as well as of avian reovirus⁵, irrespective of the nature of the host cell in which the virus was propagated¹. These molecules are synthesized while viral progeny accumulate within the cell¹; they do not appear free in the cytoplasm, but are found only in material which can be pelleted by centrifuging at 20,000g for 30 min¹ with which maturing virus particles are also known to be associated⁶, and the same is probably also true for the second class of oligonucleotides. Although the amount of A-rich oligonucleotides in reovirus is usually constant, the amount of the other oligonucleotides is about five times less in virus grown at 31° C than at 40° C⁷.