An appropriate composition of milk fatty acids (FA) improves the nutritional value of milk and milk products, and improves milk processing. Polish dairy farms in the mountainous region are rather small, animal nutrition there is based on locally produced forages and this, together with the transitional climate zone brings about seasonal changes in FA composition of milk. The aim of the study was to evaluate the composition of FA in bovine milk fat in relation to fat intake in forages and their FA profiles. The study involved 5 herds reared in low-input mountain farms located at an altitude of 670–780 m above sea level (Beskid Mountains). The cows were fed forages produced locally. FAs in forages and milk samples were subjected to gas chromatography. Highest fat intake observed in grazing season (4·2–4·7%) and high amounts of polyunsaturated FA in forages from that period (51·8–64·1 g/100 g FA) resulted in a markedly high content of valuable FAs: t-11 C18:1 (3·22 g/100 g FA), c-9, t-11 C18:2 (CLA; 1·20 g/100 g FA) in milk. Lower fat intake of forages containing high amount of SFA (32·42–38·83 g/100 g FA) in the indoor period resulted in changes in milk composition. The content of total short-chain saturated FA (SCFA) was highest in winter and early spring samples (14·10 and 13·44 g/100 g FA, respectively), like the amounts of myristic C14:0 and palmitic C16:0 acids (11·80 and 37·92 g/100 g FA). Total odd- and branched-chain fatty acids (OBCFA; 6·58 g/100 g FA) content was highest at the beginning of the grazing period. Fresh grass consumed by cows promoted the activity of Δ9-desaturase in mammary gland as evidenced by higher C14:1 : C14:0 (0·054) and C16:1 : C16:0 (0·026) ratios in grazing than in the indoor periods.