The open-burning of organo-arsenical compounds present in chemical ammunitions from the First World War was responsible for locally high concentrations of arsenic in top-soil of a highly polluted site from the region of Verdun (France). In order to understand the biogeochemistry of arsenic in this type of environment, quantitative and qualitative characteristics of microbial communities were determined in soil samples with differing As pollution levels. The total concentration of micro-organisms was negatively affected by the pollution level. However the proportion of heterotrophic As(III)-oxidizing organisms and the As(III)-oxidizing rate were higher in the most contaminated than in the less contaminated samples. These results suggest that pollutants, including arsenic, exerted a selective pressure on composition and/or activity of microbial communities.