A sensor for triggering a vehicle occupant restraint system, in particular a locking mechanism of a belt retractor, comprises an inertia body, a lower shell in which the inertia body is received, and an upper shell which rests on the inertia body and into which it projects. The upper shell is part of a pivotally mounted sensor lever which swings on displacement of the inertia body and activates the locking mechanism. At least one of the shells has at least one projecting support section for abutment of the inertia body.