It is now just 20 years since Gomatos and his co-workers at the Rocke­ feller University showed that the nucleic acid in reovirus particles is double-stranded RNA (dsRNA). This discovery created great excitement, for dsRNA was at that time under intense investigation as the replicative form of viral genomes consisting of single-stranded RNA. An equally interesting and important finding followed soon after: it was found that the reovirus genome consists, not of a single nucleic acid molecule, but of 10 discrete "segments, " each with its specific sequence content and each transcribed into its own messenger RNA. It is clear now that these segments are genes. Not surprisingly, the availability of a viral genome 10 unlinked genes has permitted some unique lines of in­ consisting of vestigation in molecular biology. Mammalian and avian reoviruses proved to be but the first of several viruses recognized as sharing Similarity in size and morphology and ge­ nomes consisting of 10, II, or 12 separate genes. These viruses are dis­ tributed throughout living organisms, among the natural hosts of mem­ bers of this virus family are vertebrates, Insects, and plants. Members of the Reoviridae family differ widely in the virulence that they exhibit toward their hosts . . For example, the first discovered mam­ malian reovirus literally is, as the name signifies, a "respiratory enteric orphan" virus, that is, a virus unassociated with disease.