This study examined the potential of liquid wild dog rose (Rosa canina) seeds oil and solid seeds residue obtained after CO₂ extraction in supercritical conditions to mitigate rumen methane production in vitro. Two experiments were carried out. The substrate comprised of a mixture of meadow hay and barley meal (60:40) for the control diets (CON1 in experiment with oil and CON2 in experiment with residue). The control diets were supplemented up to 5% in dry matter of rose seeds oil (RO) and 5% of rose seeds residue (RR). The following parameters were measured: pH, ammonia, volatile fatty acids, ciliate protozoa and bacteria count, methane concentration and methanogens population. In the RO treatment a decrease in methane production and an increase in the Archea population were observed. In the RR treatment no change in methane production was reported, whereas some variations in protozoal populations were detected in relation to CON2. The potential to mitigate methane production was reported only in wild dog rose seeds oil treatment. Besides, no negative effect of wild dog rose seeds residue on rumen processes was stated, what may predispose this protein and fibre containing by-product to be utilized as ruminants dietary ingredient.