Although plant derivatives have shown promise in reducing enteric methane (CH4) emissions from ruminants in short-term studies, investigation on possible rumen microbial adaptation to these compounds is still limited. The objective of this study was to assess the possibility of mixed rumen microbial adaptation to antimethanogenic plant derivatives over relatively long-term in vitro incubation. Treatments were: garlic oil, carvacrol and thymol, each at a final concentration of 300 mg/l.