Abstract: A reduction gear for a starter-generator of an internal combustion engine, wherein the starter-generator is functionally linked with the crankshaft (2) via a drive belt (6). A planetary gear is mounted in the force-transmitting path between the crankshaft (2) and a first belt pulley (6) that is coaxial to the crankshaft. The aim is to create a reduction gear which provides the gear ratio required for the starting mode and which can be switched in such a manner that it can be operated in any mode of operation. To this end, the reduction gear (8) is a planetary stage that includes a housing (10), an internal gear (11), a sun gear (14), a cage (15) and a planetary ring (16) with internal (18) and external (17). The internal gear intermeshes with the external teeth (17) of the planetary ring (16) and is functionally linked with the crankshaft (2) of the internal combustion engine.