UVC is well-known to impact RNA viruses negatively 30, 31 and previous studies have directly examined the mutagenic effects of UV radiation on RNA genomes. 13, 32 Furthermore, evidence from the biochemical and biophysical literature suggests that UV radiation can interfere with RNA replication fidelity. [33] [34] [35] Regardless, preliminary experiments showed that UVC exposure for periods up to 30 min greatly increased mortality in wild type phage 6 ͑Fig. 2͒, indicating that this environmental effect should produce strong selection for UVC resistance in populations of the virus. We note that identical UVC exposure was even more damaging in another well-studied virus, the ssDNA phage X174 ͑Fig. 2͒; this difference suggested that RNA phage 6 may be relatively more robust to effects of UVC than other viruses, echoing our earlier suggestion that the virus was fairly robust to mutation accumulation. 14 For this reason, experimental evolution of phage 6 under UVC exposure may provide a conservative test for whether virus populations can adapt to resist the debilitating effects of UV.