The yeast virus, ScV, is communicated only by mating. It has two separately encapsidated dsRNAs. One of these, L, codes for the major capsid polypeptide. The other, M, codes for a polypeptide toxic to yeasts without ScV-M particles. Defective interfering particles containing fragments of M (S) displace ScV-M when they arise. We have shown that five independently isolated S dsRNAs are all derived by internal deletion of M. The 3' ends of all the ScV dsRNAs are markedly heterogeneous. For instance, half of the first 35 nucleotides at one 3' end of M and S are variable. Conserved sequences at the 3' ends of M and S are AAACACCCAUCAOH and AUUUCUUUAUUUUUCAOH. Conserved sequences at the 3' ends of L are UAAAAAUUUUUCAOH and AAAAAUXCAOH, where X is variable. We propose that the sequence AUUUUUCAOH is a recognition sequence for the capsid-associated single-stranded RNA polymerase activity. Since all the viral RNAs have pppGp 5' termini, their 3' termini probably extended one nucleotide beyond the terminal pppGp.