Nocardia corallina oxidizes a variety of organic compounds including glucose, acetate, succinate, and propionate and, in each case, the amounts of oxygen used and carbon dioxide released have been less than the theoretical values for the complete oxidation of the substrate. The incubation systems contained no products of incomplete oxidation of the substrates and, with the results obtained from using uncoupling agents (sodium azide and 2,4dinitrophenol), it appeared that the low gas exchange values were due to concurrent oxidative assimilation from the substrates. Oxidative assimilation has been studied with many bacterial species and the subject has been reviewed by Clifton (1952, 1957). Barker (1936) studied the oxidation of substrates by the colorless alga Prototheca zopfii, and formulated the problem of correcting for endogenous respiration when studying microbial assimilation, manometrically. He stated that "one may take that method of correction to be most basically correct which when applied to all vessels gives the most constant value of oxygen consumption per unit quantity of substrate decomposed." The validity of subtracting endogenous values in calculating oxygen uptake figures for substrates removed by cells has been reviewed and discussed by Wilner and Clifton (1954). In general, the endogenous respiration does not appear to be suppressed during exogenous respiration (Moses and Syrett, 1955; Cochrane and Gibbs, 1951; Blumenthal et al., 1951; Santer and Ajl, 1954). This communication (a) considers the relationship between endogenous and exogenous respiration for N. corallina, (b) describes the effect of inhibitors on cellular respiration, and (c) includes evidence for oxidative assimilation from isotopically labeled substrates. METHODS The strain of N. corallina used in the investigation was isolated by enrichment culture using 9