Abstract: The invention relates to a locking and unlocking device for a gyroscopic rotor which is mounted through the intermediary of an internal cardan (5) placed at the end of a shaft (4) which receives a screwthreaded bushing (10) which is capable of being screwed onto this shaft and into the rotor (3) in order to make the assembly of rotor (3), shaft (4) and bushing (10) integral and hence to isolate the internal cardan (5) from the forces exerted upon the rotor when the latter is in the "locked" position. The bushing (10) is caused to unscrew by a spiral spring (15) to place the rotor (3) in the "unlocked" position.