Like the satellite RNA (Sat-RNA) of cucumber mosaic virus (CMV), the RNA of satellite tobacco necrosis virus (STNV-RNA) was shown to be capable of surviving in vivo without replication for at least 10 days in the absence of its helper tobacco necrosis virus (TNV). However, under similar conditions, the genomic RNA 3 of CMV failed to survive for 48 hr. It has been demonstrated that both STNV-RNA and Sat-RNA are significantly more resistant to inactivation in vitro than the RNAs of their helper viruses. The thermal denaturation kinetics of STNV-RNA and Sat-RNA, unlike those of TNV-RNA and CMV-RNA, are more like those of transfer RNA (tRNA) indicating that a high proportion of their nucleotides are involved in base pairing. STNV-RNA, Sat-RNA, and tRNA also show similar degrees of resistance to degradation by the single strand-specific S(1) nuclease. It is suggested that both STNV-RNA and Sat-RNA may owe their in vitro stability to features of their molecular structure which may also account for their ability to survive in vivo for prolonged periods without replication. Similarities and differences between satellites and viroids are discussed and it is concluded that these two classes of RNAs are unlikely to be related. The possible evolutionary origins of the satellites are also considered.