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Archaeal populations are abundant in cold and temperate environments, but little is known about their potential response to climate change-induced temperature changes. The effects of temperature on archaeal communities in unamended slurries of weakly acidic peat from Spitsbergen were studied using a combination of fluorescent in situ hybridization (FISH), 16S rRNA gene clone libraries and denaturing gradient gel electrophoresis (DGGE). A high relative abundance of active archaeal cells (11-12% of total count) was seen at low temperatures (1 and 5 degrees C), and this community was dominated by Group 1.3b Crenarchaeota and the euryarchaeal clusters rice cluster V (RC-V), and Lake Dagow sediment (LDS). Increasing temperature reduced the diversity and relative abundance of these clusters. The methanogenic community in the slurries was diverse and included representatives of Methanomicrobiales, Methanobacterium, Methanosarcina and Methanosaeta. The overall relative abundance and diversity of the methanogenic archaea increased with increasing temperature, in accordance with a strong stimulation of methane production rates. However, DGGE profiling showed that the structure of this community changed with temperature and time. While the relative abundance of some populations was affected directly by temperature, the relative abundance of other populations was controlled by indirect effects or did not respond to temperature.