Enteric methane (CH4) produced by domestic ruminants represents approximately 15% of the global emissions of this potent greenhouse gas. For reducing rumen CH4 emission various compounds have been tested as feed additives. Among these compounds, tannins are considered a promising group of natural additives. A meta-analysis by Jayanegara et al. (2012) showed that condensed and hydrolysable tannins might reduce CH4 production. However, it is still unclear (1) whether tannin supplementation reduces rumen CH4 in every situation and (2) to which extent this is associated with adverse effects on digestibility and their potential toxicity to some rumen micro-organisms (Goel et al., 2005). In this experiment we investigated the effect of tanniniferous tropical plants on enteric CH4 production and on numbers of methanogens, protozoa, and total and main cellulolytic bacteria