1. 1. Succinate production of the culture form of Trypanosoma cruzi is completely dependent on CO 2 under anaerobic conditions; under aerobic conditions CO 2 greatly enhances succinate production. 2. 2. Anaerobically, variously labeled species of glucose-C 14 are metabolized in a way consistent with the Embden-Meyerhof glycolytic scheme followed by CO 2 fixation on pyruvate, resulting in the formation of succinic acid. 3. 3. Aerobically, succinate is formed by two pathways. First, experiments with variously labeled species of glucose-C 14 indicate that a large part of the glucose is metabolized according to the Embden-Meyerhof scheme followed by the tricarboxylic acid cycle. Second, succinate is also formed by CO 2 fixation. 4. 4. Succinate formed either by tricarboxylic acid cycle reactions or by CO 2 fixation is partly excreted into the medium. Part of the succinate formed, as well as the precursor dicarboxylic acids of the second pathway mentioned above, appear to be further metabolized by the tricarboxylic acid cycle, since malonate increases the yield of succinate while causing no significant change in CO 2 production. It is therefore considered that the intermediates of both pathways are mixed in the cell.