The action ofn-butyric,n-hexanoic andiso-butyric acids on the respiration of resting cells ofCandida tropicalis was studied at pH 4.5 and 5.7 using a Warburg respirometer. In each case the rate of respiration rose as the concentration of acid was increased until a maximum was reached. After this the rate of respiration decreased until it fell to below the endogenous level. These stages occurred at much higher aliphatic acid concentrations at pH 5.7 than at pH 4.5. When the acid concentration was expressed in terms of the undissociated acid present, the curves at pH 4.5 and 5.7 became virtually coincident. Thus the respiration rate is a function of the concentration of undissociated acid present. Inhibition of the respiration occurs at a much lower concentration withn-hexanoic acid than withn-butyric acid and at a lower concentration withn-butyric acid than withiso-butyric acid. The high respiration rate observed withiso-butyric acid was unexpected, since it had been shown previously that this acid is not attacked during growth of this yeast on hydrocarbons.