This study was initiated to establish whether inhibition of growth of yeasts by medium-chain fatty acids resembled that caused by weak-acid preservatives or uncouplers. Unlike sorbic acid and 2,4-dinitrophenol, decanoic acid caused rapid cell death at its inhibitory concentration. This suggested a mode of action by medium-chain fatty acids, distinct from both weak-acid preservatives and uncouplers. Sorbic acid and 2,4-dinitrophenol both increased lag and doubling times, reduced cell yields and inhibitory concentrations of both were highly pH sensitive. The possibility is discussed as to whether weak-acid preservatives and uncouplers share common modes of inhibition.