We have analyzed the structure and diversity of bacterial biofilms formed on different water distribution materials in the absence and in the presence of 5 mg/L of arsenic(V). Three commercial metals (cast iron, zinc and copper) and polypropylene were used as substrata for biofilm formation. Biofilms were observed using scanning electron microscopy. Microbial ecology techniques (DNA amplification by polymerase chain reaction and denaturing gradient gel electrophoresis) were applied to study the bacterial community attached to each substratum. The presence of arsenic-tolerant bacteria was investigated. Microscopic observations showed that biofilm development was greater on those coupons exposed to the arsenic-containing water. The presence of arsenic also induced qualitative and quantitative changes in the planktonic bacterial community. Clustering analysis of the sessile community profiles showed that the nature of the substrata was a more important factor for the establishment of the community than the presence of arsenic in water. Culturable arsenic-tolerant bacteria were obtained from most of the biofilms. The genetic diversity of the community adapted to the presence of the contaminant was less affected by the high concentration of arsenic(V) in the culture, reflecting the presence of populations adapted to the contaminant.