A regulator assembly for fine adjustment of timepiece movements comprises an index concentrically displaceable around the axis of the balance staff, such displacement permitting a coarse regulation of the active length of the balance spring. The index is in the form of a sliding block friction fitted in a slot of an element of the movement frame such as a plate or a bridge and capable of being moved, the slot having the form of an arc of a circle concentric with the axis of the balance wheel. The sliding block bears a gudgeon having a pair of elements thereon defining a slot adapted to receive the balance spring, the gudgeon being pivotably movable parallel to the axis of the balance wheel. The elements of the pair are off-centered relative to the pivot axis of the gudgeon and co-linear with a line passing through the pivot axis, said line in the intermediate position of the gudgeon intercepting the axis of the balance staff whereby pivoting of the gudgeon enables a fine regulation of the timepiece movement.
