Pseudomonas aeruginosa colonizes the respiratory tract of cystic fibrosis patients where a high proportion of mutators along with mucoid, alginate-overproducing variants, emerge leading to a chronic infection and a poor prognosis. Mucoid conversion generally involves mutations that inactivate the mucA gene. This study correlates the activity of factors determining the mutation rate, such as MutS and the dinB-encoded DNA polymerase IV (Pol IV), with the emergence of mucoid variants. By analyzing strains deficient in mutS, dinB or both, it was observed that these two factors possess antagonistic effects on mucoid conversion, both at the quantitative and qualitative levels. While the corrective function of MutS prevented conversion to mucoidy, the error-prone activity of Pol IV enhanced the emergence frequency of this phenotype. Thus, the major incidence of mucA mutations was observed in a mutS-deficient strain, which showed mostly a -1 deletion within a track of five Gs (mucA22 alelle), a common mutation observed in CF isolates and also the most frequent kind of error reported for Pol IV. The wild type strain however, displayed a much reduced frequency of mucoid variants, and although most of them occurred via mutation in mucA, they showed a wide spectrum of mutations with a low incidence of mucA22. On the contrary, the lowest frequency of mucoid variants was obtained from the dinB strain with a proficient MMRS, and importantly, none of them became mucoid through the mucA-inactivation pathway. Taken altogether, hypermutability due to mutS deficiency and activity of Pol IV was the main source of mutagenesis in the mucA gene and it was also responsible for the major incidence of the mucA22 alelle. These results shed light on the role of MutS and Pol IV in adaptive processes through the acquisition of phenotypes such as mucoidy, a critical feature of P. aeruginosa persistence within the CF lung. At present efforts are also being made to understand the role of these two factors on other aspects of the mucoid phenotype, including its regulation and stability.