We investigated the abundance, community composition and transcriptional activity of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in the acidic fen Schlöppnerbrunnen (Germany) that was subjected to water table manipulations. Quantitative PCR targeting amoA gene copies and transcripts showed that AOA dominated the ammonia-oxidizing community in the upper 20 cm of the peat soil. Numbers of archaeal amoA gene copies and transcripts as well as the relative fraction of AOA of the total archaea decreased with depth. AOA-AmoA sequences were 96.2–98.9% identical to that of Candidatus Nitrosotalea devanaterra while bacterial AmoA sequences affiliated with Nitrosospira clusters 2 and 4. Archaeal but not bacterial amoA transcripts were detected in short-term laboratory incubations of peat that showed nitrifying activity. Nitrate accumulated in the peat pore water after 6 weeks of induced drought during a field experiment. Subsequent rewetting resulted in a significant decrease of AOA transcriptional activity, indicating that AOA responded to water table fluctuations on the transcriptional level. Our results suggest that nitrification in this fen is primarily linked to archaeal ammonia oxidation. pH and anoxia appear to be key factors regulating AOA community composition, vertical distribution and activity in acidic fens.