An anaerobic culture had been enriched with propionate, which was a critical compound inhibiting the total anaerobic process when significantly accumulated. In addition, the distribution of the trophic groups was investigated using the tool of fluorescence in situ hybridization with 16S rRNA targeted oligonucleotide probes to understand the bacterial communication. Then the kinetic constants in the competitive inhibition models were obtained. The ratio of the methanogens was increased as the culture was enriched, whereas that of the acidogens was decreased. The sum of the acetoclastic and hydrogenotrophic methanogens was 34% and the methanogens was 50% at the $68^{th}$ day. Approximately 16% of the cells detected by DAPI staining were unknown. The degradation of ethanol is thermodynamically more favorable. However, the substrate affinity to propionate was presumably higher than to ethanol since the culture had been acclimated to the propionate-rich environment. Ethanol and acetate did not show any inhibition effect on the methane fermentation of propionate.