The invention relates to an electromechanical derailleur for mounting on a rear wheel axis. The derailleur has a stationary element, a pivoting mechanism, a movable element having a chain guide arrangement, and a gearing housing. The gearing housing receives an electromechanical drive for driving the pivoting mechanism. The stationary element defines a housing void in which the gearing housing is disposed, and has a fastener for a power source. The power source is fastened to the stationary element such that said power source is electrically connected to the electromechanical drive in the gearing housing.