Cattle and kangaroos are both grazing herbivores that have developed an enlarged forestomach (the rumen in cattle), whereby anaerobic fermentation of plant material by a complex microbial consortium takes place prior to gastric digestion. During this process hydrogen is produced and must be removed. In cattle, this is done predominantly by the action of Archaea and the production of methane. Indeed, 10 – 14% of Australia’s total greenhouse gas emissions are from enteric methane emitted from ruminants. However, this is not the case with kangaroos, which emit considerably less methane, particularly from forestomach fermentation. Early studies indicated that methanogens were often below detectable levels in kangaroos but reductive acetogens were always detectable, suggesting that the dominant mechanism in kangaroo forestomach may be reductive acetogenesis, undertaken by a diversity of bacterial species. With the use of targeted isolation, PCR and gene sequencing it was shown that the Archaea present in kangaroos were very different to those in ruminants and may even include methanotrophic Archaea. Reductive acetogens were isolated that were phylogenetically distinct from previously cultivated species and were present in the 42 kangaroos examined. While good evidence for the dominance of reductive acetogenesis, this was still circumstantial. By using heavy carbon (¹³C) tracing, it has been possible to show that in rumen contents incubated with heavy CO₂ and H₂, ¹³CH₄ is rapidly produced but labelled acetate was absent, whereas the same incubation with kangaroo foregut contents resulted in heavily labelled acetate but little labelled methane, confirming the dominance of reductive acetogenesis in the kangaroo foregut. Further knowledge of the ecosystem that supports reductive acetogenesis is being compiled using comparative amplicon sequencing, metagenomics and metatranscriptomics. Both foregut microbial ecosystems are healthy, naturally occurring, ecosytems that function in a similar manner but with fundamentally different hydrogen utilising communities.