The disclosure relates to an electromechanical rear derailleur for coaxial assembling on a rear wheel axle. The derailleur has a base element, a pivot mechanism, and a movable element having a chain guide assembly. The chain guide assembly is connected to the movable element so as to be rotatable about a rotation axis. A blocking device permits the chain guide assembly to be established relative to the movable element. The blocking device is able to be inserted into the movable element in two different positions.