Complementary single-stranded RNAs from three independent VSV defective interfering particle (DI) sources examined can anneal and give rise to monomeric and multimeric circular and linear double-stranded structures observable by electron microscopy under aqueous conditions. When the RNA from the shortest of these DI is spread from 80% formamide solutions, as many as 32% of the molecules are circular, suggesting that the single-stranded RNAs contain inverted complementary terminal sequences. This is strongly supported by the isolation of the putative terminal sequences which rapidly become RNase resistant base-paired structures after melting and quick-cooling the RNA. RNase digestion yields a major and a minor component, 60 to 70 and 135 to 170 nucleotides long respectively. Snap-back DI RNAs also contain inverted complementary sequences at both ends of the plus and minus strands of the duplexes since nicking these at the ends gives rise to double-stranded molecules which can form monomeric and multimeric circular and linear molecules. Thus, snap-back molecules most likely contain a covalent linkage between or near complementary terminal sequences on the two complementary strands as schematically shown in Fig. 5D.