Syntrophic propionate oxidation (SPO) coupled with methanogenesis is often inhibited under high ammonium concentrations in anaerobic digesters. However, the inhibitory mechanism remains poorly understood. We conducted two independent laboratory experiments with a swine manure digester sludge. In experiment I, RNA-based stable isotope probing (SIP) was applied to determine the active players of both bacteria and methanogens involved in SPO under different ammonium concentrations (0, 3 and 7 g NE4+-N L-1). In experiment II, the dynamics of the bacterial community under ammonia stress was monitored using the 16S rRNA pyrosequencing and quantitative PCR under similar conditions as in experiment I but without the addition of external propionate. An additional higher ammonium treatment (10 g L-1) was applied in experiment II to maximize the ammonia stress. We identified that the Smithella bacteria and the Methanosaetaceae and Methanospirillaceae archaea were the most active players involved in SPO and methanogenesis. We revealed that Smithella, Methanosaetaceae and Methanospirillaceae were moderately and severely inhibited at 3 and 7-10 g NH4+-N L-1, respectively. However, the fermentative bacteria appeared to be more tolerant to ammonia stress. The microbial responses were corroborated with the accumulation of VFAs and the repression of methanogenesis under high ammonium conditions. (C) 2018 Elsevier Ltd. All rights reserved.