Abstract: A locking device for coupling an externally splined power take-off shaft with an internally splined coupling hub which may be part of a universal joint shaft of an agricultural implement, including locking elements which are radially movable between an inner locking position and an outer unlocking position, the elements being engaged in the locking position with a circumferential groove in the power take-off shaft. A retaining member is axially slideable on the coupling hub to a position to engage the locking elements to hold them in their radially inward locking position. Insertion of the power take-off shaft into the coupling hub causes an actuator device which includes radially movable members extending into the bore of the coupling hub to be moved radially outwardly by engagement with the power take-off shaft thereby to engage the retainer member to urge the retainer member toward its position holding the locking elements in the radially inward locking position.