The role of environmental selection in governing the structure of communities of freshwater sulfur bacteria (Achromatium spp.) was experimentally tested by mixing sediments from two geographically separated lakes (Rydal Water and Hell Kettles) that harboured Achromatium spp.. Community profiles of Achromatium spp. in sediment microcosms at day 0 and after 60 days were compared to determine whether initial Achromatium community composition or, subsequent selection by the sediment environment had greater influence in dictating the final Achromatium community structure. It was found that Achromatium spp. from the Hell Kettles community became established in mixed sediments at the expense of members of the Rydal Water community. This selection for the Hell Kettles Achromatium community was more pronounced when sediment composition was manipulated to resemble Hell Kettles sediments. Our findings definitively demonstrate that environmental selection is the primary determinant of Achromatium community structure in these sediments.