Effects of diets supplemented with fruit seed oils on fermentation parameters, ciliated protozoan population, and fatty acid composition of the rumen fluid of dairy cows were studied in 24 h batch cultures. Two diets, one containing lucerne plus wheat meal (60:40%) and the other containing of meadow hay plus wheat meal (60:40%), were supplemented with either grape oil or black currant oil (50 g · kg^–1 of dry matter). The control diet contained no oil supplementation. The oils were selected due to high content of linoleic acid (grape oil, 696 g · kg^–1 of fatty acids; black currant oil, 586 g · kg^–1 of fatty acids). Oil supplements did not affect the basal parameters of rumen fermentation. Interactions between the diets and oil supplements affected rumen methane production and either the total or the majority of rumen ciliate species examined. Although the diets had no effect on the total content of volatile fatty acids, the proportions of n-butyrate and iso-valerate were significantly affected. The concentration of polyunsaturated fatty acids was higher in the meadow hay diet than in the lucerne diet, whereas addition of oils increased polyunsaturated fatty acids content. Black currant oil supplementation proved to be more efficient in enhancing polyunsaturated fatty acids content in rumen fluid when compared to grape oil. In conclusion, both oil supplements considerably decreased methane production when lucerne was used as the diet, what could be the effect of detrimental influence of the type of diet and oil supplement on protozoa population. However, the supplements did not negatively affect other rumen parameters, hence may be considered as valuable supplements in ruminant nutrition.