A control device for a derailleur of a bicycle is described, comprising a support body, a cable-winding bush supported for rotation with respect to the support body, an indexer mechanism housed in the support body and suitable for controlling the angular position of the cable-winding bush, and a single manual actuation lever, the indexer mechanism comprising a toothed wheel integral in rotation with the cable-winding bush and having a first and a second plurality of slanting teeth, a first pawl integral with a driven arm of the lever and brought into thrusting engagement upon the first teeth while the lever is manually moved in a first direction beyond a predetermined rotation threshold, and a second pawl driven out of retention engagement with the second teeth by the driven arm of the lever while the lever is manually moved in the first direction up to the predetermined rotation threshold.