Abstract: A bicycle gear shift mechanism (8) to be actuated by an electric motor (m) has a carrier for an actuator member (S) comprising a tensioning arm (29) of a derailleur or a shift plunger of a hub gear transmission. According to the invention, the carrier comprises a cursor (27) slidably displaceable on at least two spaced parallel guides (26) mounted at stationary positions with respect to a housing (18). According to a further aspect, a control circuit for the bicycle gear shift mechanism to be actuated by a DC motor comprises a first and a second power control circuit (70,71) controlled by first and second flip-flop circuits (60, 61) the set inputs of which are connected to switches (68, 69) for shifting to higher or lower gear ratios, and the reset inputs of which are adapted to by activated via a switch (65) which is responsive to the gear-engaged state of the gear shift mechanism.