Methanobacterium formicicum has been detected previously in landfill, as an endosymbiont of the ciliate protozoan Metopus palaeformis (Finlay & Fenchel, 1991) and by oligonucleotide probe analysis to the mcrA gene on leachate samples (Maule et al., 1994). Microorganisms antigenically related to Methanobrevibacter arboriphilicus have been detected in an anaerobic sludge blanket reactor (Grotenhuis et al., 1991), but there are no reports of previous detection in landfill.