Two species of ribonuclease-sensitive Sindbis viral ribonucleic acids which sedimented at 42 S and 26 S were studied. 42 S RNA, derived either from virions or from viral nucleoids extracted from infected cultures, was converted by heating to an RNA which sedimented at 26 S . The sedimentation patterns of 42 S RNA and “derived” 26 S RNA were similarly affected in low ionic strength buffers. 42 S RNA ran as a homogeneous fraction on polyacrylamide gels; the “derived” 26 S RNA as well as “natural” 26 S RNA from infected cultures showed similar electrophoretic patterns of heterogeneity. A doubling of 3′ polynucleotide termini was observed when 42 S RNA was heated. Two possibilities concerning the structure of 42 S RNA are considered. (i) It may consist of an aggregate of subunits, joined by means of hydrogen bonds to form a complex molecule. (ii) A heat-labile covalent bond of unknown type may link viral RNA subunits. Although 26 S RNA from infected cultures and “derived” 26 S RNA from 42 S RNA behaved in a similar qualitative manner on gels, their sedimentation characteristics were affected differently in low ionic strength buffers. “Natural” and “derived” 26 S RNA appear to consist of a population of fragments. and their behavior in gradients and in gels is probably dictated by the experimental conditions of the analytical methods used.