Assays with diluted leachate supplemented with different ammonia concentrations (0.8; 1.4; 2.3; 4.4 and 8.6 g TAN.L-1) were performed with reconstituted French waste in anaerobic conditions to better understand the effect of ammonia on methane production. Micro-organisms present in the initial leachate used for the assays were acclimatized to 2 g TAN.L-1 which explains that we obtained methane inhibition only when ammonia concentration increased at 4.4 or 8.6 g TAN.L-1. Micro-organisms responsible for methane production were similar in all assays: Methanosarcinaceae. When ammonia concentration was increased twofold or fourfold compared to initial concentration, archaea were scarcely detectable using the FISH-technique. For bacteria, a noticeable decrease was noticed between 4.4 g TAN.L-1 and 8.6 g TAN.L-1.