Abstract: An anti-jackknife system for use while towing a trailer with a tractor. The tractor and trailer are coupled to each other by a fifth-wheel assembly in known manner. The fifth-wheel assembly includes a wheel plate carried by the tractor and a cooperating pin carried by the trailer, the pin being rotatable within the wheel plate when the tractor and trailer are coupled to each other. First and second abutments are carried by the trailer on opposing sides of the pin. The abutments move in paths around the pin on a change in orientation between the tractor and trailer, as during turning, for example. Fluid-actuated cylinders are carried by the tractor on opposing sides of the wheel plate with each cylinder providing a stop movable between a first position out of the abutment path. The stops are normally in the second position. A jackknife condition is detected to actuate at least one of the fluid-actuated cylinders to move its associated stop into the first position.