A device for tying cable bundles including a plastic tightening strap of indeterminate length having transverse ribs and a separate locking piece also of plastic, having an orifice for receipt of two thicknesses of the tightening strap and two tongues which project into the orifice and interact with the transverse ribs on the strap. The tongues, each of which carries at least one engaging tooth, are formed of inner walls of the locking piece opposite one another so that they can flex open to permit the strap to pass through in one direction, but which spring back to prevent the strap from passing back through in the opposite direction.
