(1) A non-sulfur purple bacterium, Rhodobacillus palustris, can utilize various fatty acids as substrates of respiration. The initial rate of respiration can be “saturated” by a very low concentrations of fatty acids, such as one several thousandsth of a mole/lit. The minimum “saturating” concentration of substrate becomes smaller as the length of carbon chain of fatty acids increases. (2) The initial rate of oxygen uptake, measured at the saturating concentration of substrate, increases with the increase of the carbon atom up to C7-C9 (Heptoic and pelargonic acids); with further increase of C, however, it decreases gradually until at C17-C18 (margalic and stearic acids) no positive O2-uptake occurs. (3) The rate of oxygen uptake caused by the addition of oxidizable fatty acids falls off abruptly to the value corresponding to endogeneous respiration when the fatty acids are used up. The amount of net oxygen uptake corresponding to the oxidation of fatty acid is directly proportional to the concentration of the acid initially added. The net oxygen uptake (in mole) per mole of fatty acid used, which is denoted by q, is the function of the number (n) of carbon atom in the acid molecule. In the case of normal fatty acids, except for acetic and propionic acid, the following linear relationship was found to hold approximately between q and n, q=0.5(n-2) This relation implies that for normal fatty acids longer than propionic acid, increase of one atom in the molecule entails increase of the net oxygen consumption by 1/2 O2. Propionic and isovaleric acids consumed 2-2.5 times as much oxygen as was predicted by this relation. On the other hand crotonic acid took up only half as much oxygen as did butyric acid. (4) The oxidation of fatty acids (tested with propionic, acetic, valeric, octanoic and isobutyric acids) seems to take place without liberation of carbon dioxide. (5) By the inhibition technique using cyanide and carbon monoxide, as well as by measuring the velocity of oxidation of para-phenylendiamine with and without butyric acid, it was shown that the oxidation of fatty acids by the organismis catalyzed by the function of cytochrome system. (6) Malonic acid, eyen in a concentration of M/100, does not exert any inhibitory action upon the oxidation of fatty acids, while monoiodoacetic acid causes about 50% inhibition in a concentration of M/10000. (7) Using Thunberg's technique it was shown that the dehydrogenase system of fatty acid oxidation cannot utilize redox-dyes such as Janus green, Nile blue, methylene blue, thionine, toluilene blue and 2, 6-dichlorophenol-indophenol, as hydrogen acceptor.