There are five families of RNA viruses in which the negative strand is sequestered in the extracellular virion. Viruses of two of these families, the Rhabdoviridae and the Paramyxoviridae, have unitary linear genomes, whereas viruses of the other three families, Arenaviridae, Bunyaviridae, and Orthomyxoviridae, have segmented genomes comprising, respectively, two, three, and seven or eight subunits. The informational macromolecules that comprise the genomes of rhabdoviruses and paramyxoviruses are among the largest functional RNA molecules and are exceeded in size only by those of the plus-strand coronaviruses. Reanney (1982Reanney ( , 1984 has calculated that the upper size limit for any RNA virus genome cannot be much in excess of 17,600 nucleotides (mol. wt. =5.7 x 10 6 ) as a consequence of the low copying fidelity of RNA polymerases. The segmentation of the genomes of the other negative-strand viruses may be a consequence of such constraints on molecular size or a device for decoupling the transcription of !ndividual genes. Whatever the reason, the genetic properties of the segmented-genome viruses differ substantially from those of the unsegmented-genome viruses, because variation in the former is generated by reassortment of genome subunits as well as by mutation. Mutation is the sole mechanism of variation in unsegmented-genome viruses, since the intramolecular recombination observed with positive-