The seasonal variations in milk fat composition, especially during the grazing period, often lead to poor eye formation in Swiss-type cheese. The influence of free fatty acids on the growth and metabolism of the dairy propionibacteria has been studied in this work. Linoleic (C 1 8 : 2 ), lauric (C 1 2 : 0 ), myristic (C 1 4 : 0 ) and oleic acids (C 1 8 : 1 ) inhibited the growth and acid production of P freudenreichii subsp shermanii in the reference medium. The antibacterial activity of linoleic acid can be overcome by additions of cholesterol and soya lecithin to the medium. The four species and two subspecies of dairyPropionibacterium could be divided into two groups according to their high susceptibility to unsaturated fatty acids (P freudenreichii subsp shermanii and subsp freudenreichii) or low susceptibility (P acidipropionici, P jensenii and P thoenii). Nevertheless, this inhibitory action of free fatty acids was not observed in milk, retentate media or lactic curd. A total extraction of the milk lipids led to the recovery of this inhibitory effect on Propionibacterium. The possible modes of action of these molecules are discussed on the basis of the observed potassium effluxes and disturbances in the cell membranes.