Propionate degradation is crucial for maintaining the efficiency and stability of an anaerobic reactor. However, there was little information about the effects of ecological factor on propionate-oxidizing bacteria (POB). In current research, quantitative real-time fluorescence polymerase chain reaction (QPCR) of some identified POB and methanogens with a decrease in temperature in an upflow anaerobic sludge bed (UASB) reactor containing propionate as sole carbon source was investigated. The results showed that there were at least four identified POB, including Pelotomaculum schinkii, Pelotomaculum propionicum, Syntrophobacter fumaroxidans, and Syntrophobacter sulfatireducens, observed in this UASB reactor. Among them, P. schinkii was dominated during the whole operational period. Its quantity was 1.2 x 10(4) 16S rRNA gene copies per nanogram of DNA at 35 A degrees C. A decrease in temperature from 35 to 30 A degrees C led to P. schinkii to be increased by 1.8 times and then it was gradually reduced with a decrease in temperature from 30 to 25, 20, and 18 A degrees C stepwise. A decrease in temperature from 35 to 20 A degrees C did not make the amount of methanogens markedly changed, but hydrogenotrophic methanogens (Methanospirillum) and acetotrophic methanogens (Methanosaeta) at 18 A degrees C were increased by an order of magnitude and 1.0 time, respectively, compared with other experimental conditions.