The double-stranded RNAs from bacteriophage phi6 and the replicative form of mengovirus denature upon heating in a series of abrupt steps which resemble the subtransitions (thermalites) observed within the high resolution profiles of small, naturally occurring DNA molecules. Such RNA thermalites are approximately an order of magnitude narrower than typical thermal subtransitions of nominally single-stranded RNA. We conclude that the same features of nucleotide sequence that give rise to cooperative denaturation in DNA genomes are to be found also in RNA genomes. Thus, high resolution thermal denaturation profiles are useful for characterizing double-stranded RNA molecules as well as native DNA in the size range of common viruses. A medium containing dimethylsulfoxide was required to lower the Tm of the RNA samples to a satisfactory temperature range. For double-stranded RNA in 50% dimethylsulfoxide, the dependence of Tm on G . C composition was greater than that of DNA in the same medium and also greater than that of double-stranded RNA in an aqueous medium. The fact that RNA thermalites are broader than DNA thermalites and that the melting temperature of double-stranded RNA has a greater dependence on base composition than that of DNA, indicates that at least one of the thermodynamic parameters for double helix formation in RNA is different from that in DNA.