Electromechanical gearshift mechanism (10, 200) for coaxial mounting on a rear-wheel axle of a bicycle, having: - a base element (20, 220) for releasable mounting on a bicycle frame, - a movable element (40, 240), - a chain-guiding arrangement which is connected to the movable element (40, 240) in a manner rotatable about a rotary axle P, - a pivot mechanism (30, 230) which connects the base element (20, 120, 220) to the movable element (40, 240) in such a way that the movable element (40, 140, 240) is movable relative to the base element (20, 220), and - an electromechanical drive (80, 250) for driving the pivot mechanism (30, 230), wherein the base element (20, 220) comprises a first attachment end (21, 221) for coaxial mounting on the rear-wheel axle, and a second attachment end (29, 229) for coupling to the pivot mechanism, wherein the first attachment end of the base element has a first arm and a second arm, which are arranged spaced apart from one another in an axial direction, and the first arm has a first centring opening and the second arm has a second centring opening, characterized in that the electromechanical drive (80, 250) is accommodated in a transmission housing, and the base element (20, 220) defines a housing free space in which the transmission housing is at least partially arranged, wherein the base element is of multi-part form and comprises at least a first base sub-element for mounting on a bicycle frame and a second base sub-element for mounting on the first base sub-element, and the housing free space is defined between the first base sub-element and the second base sub-element, wherein the first base sub-element comprises the first attachment end of the base element, which has the first arm and the second arm. - a base element (20, 220) for releasable mounting on a bicycle frame, - a movable element (40, 240), - a chain-guiding arrangement which is connected to the movable element (40, 240) in a manner rotatable about a rotary axle P, - a pivot mechanism (30, 230) which connects the base element (20, 120, 220) to the movable element (40, 240) in such a way that the movable element (40, 140, 240) is movable relative to the base element (20, 220), and - an electromechanical drive (80, 250) for driving the pivot mechanism (30, 230), wherein the base element (20, 220) comprises a first attachment end (21, 221) for coaxial mounting on the rear-wheel axle, and a second attachment end (29, 229) for coupling to the pivot mechanism, wherein the first attachment end of the base element has a first arm and a second arm, which are arranged spaced apart from one another in an axial direction, and the first arm has a first centring opening and the second arm has a second centring opening, characterized in that the electromechanical drive (80, 250) is accommodated in a transmission housing, and the base element (20, 220) defines a housing free space in which the transmission housing is at least partially arranged, wherein the base element is of multi-part form and comprises at least a first base sub-element for mounting on a bicycle frame and a second base sub-element for mounting on the first base sub-element, and the housing free space is defined between the first base sub-element and the second base sub-element, wherein the first base sub-element comprises the first attachment end of the base element, which has the first arm and the second arm. wherein the base element (20, 220) comprises a first attachment end (21, 221) for coaxial mounting on the rear-wheel axle, and a second attachment end (29, 229) for coupling to the pivot mechanism, wherein the first attachment end of the base element has a first arm and a second arm, which are arranged spaced apart from one another in an axial direction, and the first arm has a first centring opening and the second arm has a second centring opening, characterized in that the electromechanical drive (80, 250) is accommodated in a transmission housing, and the base element (20, 220) defines a housing free space in which the transmission housing is at least partially arranged, wherein the base element is of multi-part form and comprises at least a first base sub-element for mounting on a bicycle frame and a second base sub-element for mounting on the first base sub-element, and the housing free space is defined between the first base sub-element and the second base sub-element, wherein the first base sub-element comprises the first attachment end of the base element, which has the first arm and the second arm.