Methanogenic communities in two identical Upflow Anaerobic Sludge Bed Reactors (UASBR) seeded with the same sludge and operated at psychrophilic and mesophilic temperatures were comparatively investigated by using Fluorescent In-Situ Hybridization (FISH) technique. Furthermore, effects of temperature on the reactor performances were correlated with population dynamics. initially, until steady state conditions, synthetic wastewater and then raw domestic sewage were fed to the reactors. Archaeal and bacterial communities in the reactors were investigated by using Cy-3 and Fluorescein labelled rRNA targeted oligonucleotide probes for Bacteria (Eub338) and Archaea (Arch915) domains and for Methanosaeta (MX825) and Methanosarcina (MS821) species as acetoclastic methanogens. According to in-situ hybridization results with Arch915 and Eub338 probes, archaeal cells representing. the methanogens were found intensively dominant in the bottom sampling ports of the reactors fed with synthetic wastewaters at all sampling days. Besides, Methanosaeta sp. (MX825) abundant in seed sludge was identified as the major methanogenic archaea in samples taken from both psychrophilic and mesophilic reactors as granules remained undisturbed and kept their rigidity even under psychrophilic conditions. On the other hand, this is an indication of temperature adaptation for originally mesophilic Methanosaeta species in psychrophilic reactor. Comparing the results of FISH experiments for the anaerobic treatability studies with synthetic wastewater and raw domestic sewage the microbial diversity did not significantly change in both of the reactors, while COD removal efficiencies dropped dramatically.