RNA viruses have long been known from anecdotal evidence to mutate at high rates, but few rate measurements have been performed, and none accurately. Simple theory was constructed to convert mutant frequencies into mutation rates in both lytic and retroviral systems employing RNA genomes. A set of mutant frequencies was discovered in the literature that could be converted into mutation rates. The median rate for several lytic viruses (phage O beta poliovirus, influenze A and vesicular stomatitis viruses) was in the neighborhood of 1 per genome pre replication. Thus, these organisms have mutation rates about as large as is consistent with survival. Three retroviruses exhibited rates several-fold smaller, but still far higher than for DNA-based microbes.