A gearshift control device for a chain and sprocket transmission system including a gearshift member that is movable along a transverse direction relative to the plane defined by the chain, a shaft, a first driving element having a drive member that is non-centered or non-concentric with respect to the axis of the shaft, a second positioning element of the derailleur with an indexing device, a third control element mounted to rotate freely about the shaft with a blocking device, and a transmission element having a member for connection with the first driving element, the second positioning element and the third control element, the transmission element being operably between the first driving element, the second positioning element, and the third control element.