Svalbard reindeer are isolated on the high-arctic archipelago of Svalbard under austere nutritional conditions, while the semi-domesticated Norwegian reindeer migrate between lush coastal summer pastures and inland taiga and boreal forest habitats with ground lichens in winter on mainland Norway. We propose that interactions between reindeer and Arctic plants through 15 million years of evolution have resulted in a unique rumen microbial ecosystem, and present data on rumen bacterial diversity analysed by PCR amplification and sequencing of 16S rDNA clone libraries from these sub-species of reindeer. Resulting sequences represent novel bacteria different from those reported in other ruminants.