This paper examines the impact of the seed source on methanogenic abundance and overall performance of thermophilic anaerobic digesters treating the source-sorted organic fraction of municipal solid waste (SS-OFMSW). Two digesters were started up with different seed mixes and operated for 250 days until steady-state was reached. Quantitative Polymerase Chain Reaction (QPCR) was used to determine the total number of Bacteria and the three main methanogenic orders: Methanobacteriales, Methanomicrobiales and Methanosarcinales (Methanosarcinaceae and Methanosaetaceae). Digester A was inoculated with a mix of digested waste and cattle manure, both of which are predominated by the acetotrophic methanogens M.sarcinaceae (95% and 85%, respectively), along with waste activated sludge. The latter presents equal abundance of hydrogenotrophic M.microbiales (50%) and acetotrophic M.saetaceae (46%) both of which are absent in the other 2 seeds. Digester B was seeded with the same mix as digester A with the addition of landfill leachate (94% M.microbiales) and municipal waste compost (41% M.sarcinaceae, 37% M. saearaceae and 22% M.bacteriales). The addition of compost and leachate induced an increase in the number of predominant orders (M.sarcinaceae and M.bacteriales) as well as minor ones (M. sarcinaceae and M. microbiales). With time, this impact was attenuated upon reaching steady-state with no significant differences in methane generation and biogas composition. Accordingly, the initial seed seems to have minor impact on the ultimate methanogenic composition and abundance, with environmental conditions (such as type of feed, temperature, mixing scheme, feeding mode and frequency) being the determinant factors. Ultimately, the digester with added compost and leachate exhibited lower VFAs (55%) and COD (27%) levels at steady-state which can be attributed to potential differences in composition (and function) of the bacterial flora.