An instrument for inserting an intervertebral prosthesis, comprising two prosthesis holders which are connected by a parellel guide and can be spread apart from one another and are intended to receive a pair of prosthesis plates. The first prosthesis holder is arranged fixedly on an elongate instrument body. The second prosthesis holder is held from the instrument body by means of a parallel guide. The parts connecting the second prosthesis holder to the instrument body or to the first prosthesis holder delimit on both sides a central through-opening which extends like a channel in the longitudinal direction of the instrument body and whose width corresponds at least to the transverse dimensions of a prosthesis core, to be inserted between the prosthesis plates, and of the prosthesis core holder provided for this.