Northern peatlands play a major role in the global carbon cycle as sinks for CO(2) and as sources of CH(4). These diverse ecosystems develop through accumulation of partially decomposed plant material as peat. With increasing depth, peat becomes more and more recalcitrant due to its longer exposure to decomposing processes. Compared with surface peat, deeper peat sediments remain microbiologically poorly described. We detected active archaeal communities even in the deep bottom layers (-220/-280 cm) of two Finnish fen-type peatlands by 16S rRNA-based terminal restriction fragment length polymorphism analysis. In the sediments of the northern study site, all detected archaea were methanogens with Rice Cluster II (RC-II) and Methanosaetaceae as major groups. In southern peatland, Crenarchaeota of a rare unidentified cluster were present together with mainly RC-II methanogens. RNA profiles showed a larger archaeal diversity than DNA-based community profiles, suggesting that small but active populations were better visualized with rRNA. In addition, potential methane production measurements indicated methanogenic activity throughout the vertical peat profiles.