| INTRODUC TI ONWild ruminant animals are excellent models for the functional surveillance of rumen microbes (Kobayashi, 2006), since these animals are expected to harbor specific rumen microbes in response to the dietary conditions to which they are exposed. Changes of rumen microbiota likely depend on the area in which the wild animals inhabit. In particular, harsh dietary conditions might select for rumen microbiota to digest and ferment fibrous materials as a main dietary ingredient in order to maximize nutrient extraction. This is because the rumen is the main digestive organ for degrading fibrous foods by microbes and also acts as a reservoir of ingested foods (Itabashi, 1998).Hokkaido, the northernmost island of Japan, is home to more than a million head of wild sika deer (Hokkaido sika deer, Cervus nippon yesoensis), many of which live in snowy areas with limited access to food. We have elucidated seasonal changes of body size, ingesta, body fat reserve, and rumen fermentation products of sika deer in the Shiretoko Peninsula of Hokkaido, characterized by severe winter conditions (long duration of low temperatures AbstractRumen digesta was obtained from wild Hokkaido sika deer to compare bacterial flora between summer and winter. Bacterial flora was characterized with molecular-based approaches and enrichment cultivation. Bacteroidetes was shown as a major phylum followed by Firmicutes, with similar proportions in both seasons. However, two phylogenetically unique groups in Bacteroidetes were found in each season: unknown group A in winter and unknown group B in summer. The ruminal abundance of unknown group A was the highest followed by Ruminococcus flavefaciens in winter.Moreover, the abundance of these two was higher in winter than in summer. In contrast, the abundance of unknown group B was higher in summer than in winter. In addition, this group showed the highest abundance in summer among the bacteria quantified. Unknown group A was successfully enriched by cultivating with oak bark and sterilized rumen fluid, particularly that from deer. Bacteria of this group were distributed in association with the solid rather than the liquid rumen fraction, and were detected as small cocci. Accordingly, unknown group A is assumed to be involved in degradation of fibrous materials. These results suggest that wild Hokkaido sika deer develop a rumen bacterial flora in response to changes in dietary conditions. K E Y W O R D Secology, fiber digestion, quantitation, rumen bacteria, sika deer