The diversity and ecology of natural communities of the uncultivated bacterium Achromatium oxaliferum were studied by use of culture-independent approaches. 16S rRNA gene sequences were PCR amplified from DNA extracted from highly purified preparations of cells that were morphologically identified as A. oxaliferum present in freshwater sediments from three locations in northern England (Rydal Water, Jenny Dam, Hell Kettles). Cloning and sequence analysis of the PCR-amplified 16S rRNA genes revealed that multiple related but divergent sequences were routinely obtained from the A. oxaliferum communities present in all the sediments examined. Whole-cell in situ hybridization with combinations of fluorescence-labelled oligonucleotide probes revealed that the divergent sequences recovered from purified A. oxaliferum cells corresponded to genetically distinct Achromatium subpopulations. Analysis of the cell size distribution of the genetically distinct subpopulations demonstrated that each was also morphologically distinct. Furthermore, there was a high degree of endemism in the Achromatium sequences recovered from different sediments; identical sequences were never recovered from different sampling locations. In addition to ecological differences that were apparent between Achromatium communities from different freshwater sediments, the distribution of different subpopulations of Achromatium in relation to sediment redox profiles indicated that the genetically and morphologically distinct organisms that coexisted in a single sediment were also ecologically distinct and were adapted to different redox conditions. This result suggests that Achromatium populations have undergone adaptive radiation and that the divergent Achromatium species occupy different niches in the sediments which they inhabit.