We constructed a mesophilic anaerobic chemostat that was continuously fed with synthetic wastewater containing propionate as the sole source of carbon and energy. Steady-state conditions were achieved below the critical dilution rate of 0.3 d (-1) with almost complete substrate degradation. The propionate-degrading methanogenic communities in the chemostat at dilution rates of 0.01, 0.08, and 0.3 d (-1) were analyzed using molecular biological techniques. Fluorescence in situ hybridization with archaeal and bacterial domain-specific probes showed that archaeal cells predominated throughout the three dilution rates. Archaeal-16S rRNA gene clone library analysis and quantitative real-time polymerase chain reaction studies showed that hydrogenotrophic methanogen rRNA genes closely related to Methanoculleus was detected at a dilution rate of 0.01 d (-1) , whereas rRNA genes closely related to the Methanoculleus and Methanospirillum genera were detected at dilution rates of 0.08 and 0.3 d (-1) . The aceticlastic methanogen, Methanosaeta , was detected throughout the three dilution rates. Bacterial-rRNA gene clone library analysis and denaturing gradient gel electrophoresis demonstrated that rRNA genes affiliated with the genus Syntrophobacter predominated at the low dilution rate, whereas rRNA genes affiliated with the phylum Firmicutes predominated at the higher dilution rates. A significant number of rRNA genes affiliated with the genus Pelotomaculum were detected at dilution rate of 0.3 d (-1) . The diversity of genes encoding acetate kinase agreed closely with the results of the rRNA gene analysis. The dilution rates significantly altered the archaeal and bacterial communities in the propionate-fed chemostat.