A naturally competent Acinetobactercalcoaceticus system has been developed to investigate the frequency of gene exchange by natural transformation in drinking water and biofilms. We report here 1) the first demonstration of bacterial transformation in drinking water, with and without disinfectant residual (respective transformation frequencies: 6.59×10−7 and 8.81×10−7), 2) that disinfectant residual had no effect on the ability of these bacteria to exchange plasmid resident genes and 3) the development of a bacterial system to characterize natural transformation in biofilms.