There is an increasing societal demand for research involving enteric methane (CH4) emissions from ruminant livestock. Dairy cows are particularly important contributor to the emissions in Sweden. Enteric CH4 emissions in lactating dairy cows vary. Variation is attributed to both animal and dietary factors. Current understanding of the biological basis of animal-to-animal variation in ruminal CH4 production is limited. Therefore, the aim of this project is to elucidate the role of specific ruminal factors contributing to the observed levels of CH4 production. In this project we will perform both direct measurements on lactating cows as well as analyze rumen fermentation data from an existing, large dataset (meta-analysis). With respect to the direct measurements, the objective is to rank the cows based on the level of CH4 production and to correlate the level of CH4 with cow´s ability to digest fibre and the fatty acid profile in milk. Another, more specific objective is to characterize the high and low CH4 emitters with respect to rumen microbial composition and ruminal fermentation profiles. Expected project outcomes include improved understanding of the nature of animal variation in CH4 production with respect to associated effects on fiber digestibility and assessment of specific fatty acids of bacterial origin as biomarkers of low CH4 phenotype. Detailed study on microbial ecology in the rumen will further improve our understanding of animal variation in CH4 production.