Abstract: A process for the preparation of a dicarboxylic acid using a thermophilic croorganism, and the microorganism used. The process involves cultivating a microorganism belonging to the genus Mycobacterium which produces a dicarboxylic acid at high temperatures in a medium to which a substrate selected from among normal paraffins, fatty acids, and their derivatives, each containing 6 to 22 carbon atoms, has been added, so that a dicarboxylic acid containing 6 to 22 carbon atoms is formed and accumulated in the medium, and collecting the dicarboxylic acid.