Abstract: A motorcycle antitheft system in which the motorcycle engine is rendered inoperable by mechanical restraint of the crankshaft. The crankshaft is housed in a crankcase which has two sections held together by bolts positioned in recesses in one of the sections. The antitheft system comprises a bore formed in the crankcase, a lock assembly mounted in the bore for axial movement therein between a locked position and an unlocked position, a rigid plunger secured to one end of the lock assembly extending toward the crankshaft and a toothed crankwheel mounted on the crankshaft. When the lock assembly is in the locked position, it passes through one of the bolt recesses in the crankcase, thereby blocking access to the bolt and preventing opening of the crankcase. Movement of the assembly to the locked position also extends the plunger into a space between teeth on the crankwheel to thereby prevent rotation of the crankwheel and crankshaft.