The effect of 5% rapeseed, sunflower or linseed oil supplementation to a high-concentrate diet on basic rumen parameters, methane emission, rumen bacteria and protozoa counts was estimated in vitro in batch culture studies. The inclusion oils differing in fatty acid composition did not affect the pH or ammonia level of rumen contents but decreased VFA and butyric acid levels (P≤0.05), whereas the level of particular VFAs and the acetate-to-propionate ratio were not influenced by different oils. Protozoa number was significantly decreased by all supplemented oils, whereas no effect of fat on bacteria count was noted. As the number of unsaturated bonds in the supplemented oils increased, the protozoa counts decreased. Each oil only slightly decreased in vitro dry matter disappearance (P>0.05). Methane emission was reduced (P≤0.01) when oils were added to the incubates.