Abstract: An adjustable chin strap for motorcycle helmets, including two strap portions that are interconnected via a self-clamping, double-acting, retaining roller closure mechanism, with the closure mechanism comprising a frame having at least two retaining rollers, and on one or both short, transversely disposed parts of the frame of the closure mechanism an adjustment flap is connected so as to be pivotable about the frame part, with the span of the adjustment flap in the direction of pulling of the strap corresponding to the desired change in head size of the strap, with the adjustment flap being pivotable into the interior of the frame of the closure mechanism.