The replication error rate is one of the main factors affecting the evolution of populations. There are evidences that this is not a constant parameter, as it can be modified in response to the variability of the environment. In contrast to the frequent selection of hypermutator bacteria, there are no reported examples of natural variants of RNA viruses with error rates above the standard. Possibly RNA viruses replicate their genomes with the maximal error rate compatible with the maintaining of the genetic information, and additional increases in this parameter seriously compromise their viability. Nevertheless, the error rate can be increased artificially with mutagens, an approach that has been successfully used to extinguish virus populations. In this contribution we present a study of the evolution of bacteriophage Qbeta when it replicates through successive passages in the presence of increasing concentrations of the chemical mutagen 5-azacytidine (AZC). Initially, the treatment with AZC causes an increase in the mutation frequency, accompanied by a decrease in the virus yield. The analysis of how mutations distribute along the genome shows that they are located preferably at certain regions, suggesting that the selective processes can still control the mutational pressure. After additional passages, in presence of higher concentrations of mutagen, an AZC- resistant population is selected. The phenotype of this population is characterized by the stabilization of the mutation frequency and by an increase in the virus yield in the presence of AZC. The consensus sequence reveals the presence of several mutations, some of them located in the replicase gene. The results showing the possible molecular mechanisms by which these mutations can confer AZC resistance will be discussed in the presentation.