RNA interference (RNAi) systems were lost from the ancestors of some species of yeast but not from the ancestors of others. The authors investigated the consequences of restoring a functional RNAi system to Saccharomyces cerevisiae by introducing the genes encoding Argonaute and Dicer. Introducing RNAi interfered with the maintenance of killer, an inherited viral system consisting of two double-stranded RNAs, M and L-A. M encodes a toxin that kills neighbouring cells but confers immunity to producing cells, and L-A is required to maintain M. In the RNAi-reconstituted strain, M and L-A were processed by Argonaute and Dicer into small interfering RNAs and were then lost from the cells. In all yeast species analyzed, the presence of killer coincided with the absence of an RNAi system. These findings suggest that RNAi provides less of a selective advantage to some species than a killer system, driving its loss in certain fungal lineages.