The phylogenetic and functional diversity of syntrophic prop ionate-oxidizing bacteria (POB) present in an anaerobic digester was investigated by microautoradiography combined with fluorescent in situ hybridization (MAR-FISH) that can directly link 16S rRNA phylogeny with in situ metabolic function. The syntrophic POB community in the anaerobic digester sludge consisted of at least four phylogenetic groups: Syntrophobacter, uncultured short rod Smithella (Smithella sp. SR), uncultured long rod Smithella (Smithella sp. LR), and an unidentified group. The activities of these POB groups were dependent on the propionate concentrations. The uncultured Smithella sp. SR accounted for 52-62% of the total active POB under all the propionate concentrations tested (0.515 mM). In contrast, uncultured Smithella sp. LR was active only at lower propionate concentrations and became a dominant active POB at 0.5 mM of propionate. Syntrophobactet- accounted for 16-3 1 % of the total active POB above 2.5 mM propionate, whereas the active Syntrophobacter population became low (ca. 6%) at 0.5 mM of propionate. The anaerobic digester was operated in a fill and draw mode, resulting in periodical changes in propionate concentration ranging from 0 to 10 mM. These phylogenetically and functionally diverse, to some extent functionally redundant, active POB communities were dynamically responding to the periodical changes in propionate concentration.