A ratio selector mechanism has a first fluid pressure operated actuator (14) which moves the selector member (10) in a first direction to engage one of a number of ratio shift rails (11,12,13) and a second fluid pressure operated actuator (15) which moves the selector member (10) in a second direction to longitudinally displace the engaged shift rail (11,12,13) to engage an associated selector ratio, the mechanism including a valve means which is associated with the first actuator (14) and varies the pressure of fluid to the second actuator (15) in accordance with the position of the first actuator (14).