This study evaluated concentrations of archaeol, a lipid biomarker for Archaea and, presumably, methanogens in cattle faeces. Twelve continental cross-bred steers were allocated at random to receive either a concentrate based diet or a grass silage based diet. Lipids were extracted from dried faeces, separated into specific compound classes and analysed by gas chromatography/mass spectrometry, and CH(4) production was estimated using the sulphur hexafluoride (SF(6)) technique. Steers fed the grass silage based diet consumed less feed (9.15 versus 11.43 kg dry matter (DM)/d: SED=0.824; P<0.05), emitted more CH(4) (341 versus 174 g/d; SED = 60.5; P<0.05) and produced faeces with higher concentrations of archaeol (30.6 versus 5.1 mg/kg DM; SED = 5.42; P<0.001) than those fed the concentrate based diet. Other dialkyl glycerol lipids, such as hydroxyarchaeol, macrocyclic archaeol and unsaturated archaeol analogues, were not detected. Since the feeds contained no archaeol, it must have been produced in the ruminant digestive tract, most likely by Archaea in the rumen. The relationship between CH(4) emission and faecal archaeol concentration for individual animals was weak. Possible explanations include the inherent limitations of the SF6 technique, the sampling regime, selective retention of Archaea in the rumen and possible degradation of the archaeol during gut transit and post excretion. Despite these limitations, faecal lipid biomarker analysis shows potential as a new method to study ruminant methanogens.