The aim of the present study was to investigate the structure, phenotypic diversification and competitiveness of Pseudomonas aeruginosa hypermutator biofilms. For this purpose, the environmental strain Hex1T and its respective mutS mutant, Hex1TMS, were grown in flow-cell biofilms and inspected by confocal laser scanning microscopy. Structural analyses revealed that biofilms formed by Hex1TMS strain were similar to those formed by Hex1T strain, displaying the characteristic mushroom-shaped structures of P. aeruginosa biofilms. However, hypermutator biofilms showed significantly increased phenotypic diversity in comparison to wild type biofilms, with a Shannon-Weaver Index (H) of 0.31 and 0.03, respectively (P<0.0001). Normal, giant, small, wrinkly, transparent and brown pigmented were among the different morphotypes found. Competition experiments between Hex1T and Hex1TMS strains with initial ratios 1:1 and 1:100 showed that the hypermutable strain had increased fitness after 6 days, indicating a potential adaptive advantage of hypermutators during biofilm mode of growth. The fact that this increased competitiveness was afforded by phenotypic diversification was confirmed by competing the principal evolved morphotypes with the parental mutS strain. These results give further evidences of how hypermutator bacteria can accelerate the speed of evolution of a bacterial population.