The photosynthetic bacterium, Rhodospirillium rubrum, requires carbon dioxide for aerobic growth in the dark in a medium containing acetate. Resting cells do not oxidize acetate in the dark in the absence CO 2. Dark oxidation of acetate is stimulated by catalytic amounts of malate;Rhodospirillium rubrum when grown in a medium containing malate in the presence of radioactive carbon dioxide yields the following radioactive amino acids: aspartate, glutamate, arginine, tryptophan, proline, threonine, glycine, leucines, tyrosine, serine, methionine, valine, cystine, and lysine; hydroxyproline is tentatively identified. Aspartate, glutamate, alanine, and arginine account for 45 percent of the activity in the cationic fraction of the hydrolysate;Alanine, aspartate, and glutamate are labeled predominantly in carboxyl groups. The labeling is consistent with a C1-C2 addition to form pyruvate; beta-carboxylation of pyruvate yielding oxalacetate, and a C2-oxalacetate condensation to form citrate. Citrate would be converted to a alpha-ketoglutarate by reactions of the citric acid cycle. It is suggested that the amino acids corresponding to pyruvate in this organism. Cell-free extracts catalyze the formation of glutamate from aspartate and pyruvate. The scheme presented above is in accord with this observation;The amidine carbon of arginine contains 33 percent of the activity in arginine;The formation of various amino acids from alanine, aspartate, glutamate and members of the citric acid cycle is discussed in relation to recent studies on the biosynthesis of amino acids in microorganisms;Whole cells and cell-free extracts of Rhodospirillium rubrum , grown in the dark or light, contain transaminases which catalyze the formation of glutamate from alpha-ketoglutarate and the following L-amino acids: phenylalanine, aspartate, tyrosine, ornithine, valine, leucine, isoleucine, tryptophan, histidine and alanine. Methionine and lysine are slightly active and NH4Cl, proline, hydroxyproline, and glycine were ineffective as amino donors. The aspartate-alpha-ketoglutarate and phenylalanine-alpha-ketoglutarate reactions are reversible. Transaminases are probably involved in the biosynthesis of amino acids in Rhodospirillium rubrum;D-Glutamate-oxalacetate and D-glutamate-pyruvate transamination reactions are catalyzed by a cell-free extract. The D-amino acid transminase activity is not caused by L-isomers in the preparation of the D-amino acid and is not caused by glutamic acid racemase activity in the cell-free extract. The possible role of these reactions in the metabolism of this organism is unknown;An air-borne radioactive material was found which caused significant errors in tracer experiments. The characteristics of the radiation were investigated and a procedure developed for avoiding errors from this source.