Abstract: The three families of double-stranded RNA (dsRNA) viruses and two families of retroviruses (retrotransposons) of the yeast Saccharomyces cerevisiae are all transmitted between cells only by cell fusion, probably reflecting the high frequency of mating of yeast cells in nature. One dsRNA virus and two retroviruses apparently use ribosomal "frameshifting" to produce major coat protein-polymerase fusion proteins. This mechanism allows regulation of the relative amounts of major coat protein and fusion protein that are made and avoids the possibility of mutant virus genomes being generated by splicing. Moreover, the fusion protein structure suggests a possible mechanism of genome packaging. The recent development of in vitro replication, transcription, and integration systems for these viruses, and the ease with which classical genetic and molecular studies are executed in yeast, are yielding detailed information about the roles of cellular and viral components in the viral replication cycles and the host defensive response. A host defense system against yeast dsRNA viruses is known and there is evidence to suggest a system active against the retroviruses.