Two experiments were conducted to study the effect of the consumption of different amounts of immature Pyrenean oak leaves (OL) by beef cattle on in vitro ruminal fermentation and potential adaptation of rumen microorganisms. A total of twelve ruminally cannulated young Brown Swiss bulls were divided in experimental groups that received different amounts of OL via the ruminal cannula (on average 0, 2.5, 5.2 and almost 10 kg fresh matter per animal and day). The gas production technique was used to study in vitro fermentation of two substrates (grass hay and OL) incubated with rumen inocula derived from each bull. Results suggest not only a dose-dependent negative effect of tannins consumption on ruminal fermentation of common feeds (e.g., grass hay), but also an adaptation of rumen microbial populations from animals receiving moderate amounts of OL. The high level of tannins in the rumen of bulls that received the highest amount of OL would have exceeded the ability of microorganisms to tolerate or detoxify them.