The effects of varying concentrations of 2,4-dinitrophenol on the oxidation of a series of saturated, aliphatic fatty acids byPs. aeruginosa were studied. The oxidation of C4, C6, C8, C10, C12, C13, C14 and C16 acids is stimulated by certain concentrations of this inhibitor. However, the concentration of 2,4-dinitrophenol which causes stimulation of oxygen uptake with capric acid does not produce an increase in numbers of the organisms in a medium containing the fatty acid as the sole carbon source.