Methane (CH(4)) emissions from boreal wetlands show considerable seasonal variation, including small winter emissions. We addressed the seasonality of CH(4)-producing microbes by comparing archaeal communities and the rates and temperature response of CH(4) production in a boreal fen at three key phases of growing season and in winter. Archaeal community analysis by terminal restriction fragment length polymorphism and cloning of 16S ribosomal DNA and reverse-transcribed RNA revealed slight community shifts with season. The main archaeal groups remained the same throughout the year and were Methanosarcinaceae, Rice cluster II and Methanomicrobiales-associated Fen cluster. These methanogens and the crenarchaeal groups 1.1c and 1.3 were detected from DNA and RNA, but the family Methanosaetaceae was detected only from RNA. Differences between DNA- and RNA-based results suggested higher stability of DNA-derived communities and better representation of the active CH(4) producers in RNA. Methane production potential, measured as formation of CH(4) in anoxic laboratory incubations, showed prominent seasonality. The potential was strikingly highest in winter, possibly due to accumulation of methanogenic substrates, and maximal CH(4) production was observed at ca. 30 degrees C. Archaeal community size, determined by quantitative PCR, remained similar from winter to summer. Low production potential in late summer after a water level draw-down suggested diminished activity due to oxygen exposure. Our results indicated that archaeal community composition and size in the boreal fen varied only slightly despite the large fluctuations of methanogenic potential. Detection of mRNA of the methanogenic mcrA gene confirmed activity of methanogens in winter, accounting for previously reported winter CH(4) emissions.