Abstract: A safety device for use with the fifth wheel and kingpin of a tractor-semitrailer combination, in which an auxiliary pin is insertable through an aperture in the fifth wheel upwardly into an arcuate slot located on the underside of the semitrailer and concentric with the kingpin. The auxiliary pin is mounted in a carrier frame fixed to the underside of the fifth wheel. A sleeve is mounted on the carrier frame in axial alignment with the aperture in the fifth wheel, the auxiliary pin being slidable axially in the sleeve and having an arcuate cam groove in its cylindrical surface, an inwardly projecting follower pin of the sleeve engaging the arcuate groove. The auxiliary pin is advanced into, and retracted from, the arcuate slot and at the same time the interaction between the follower pin and the cam groove.