A collimator for a computer tomograph includes a number of collimator plates and a holder for the collimator plates. The collimator plates include, at the edge, a number of tabs spaced apart from one another. In a fashion corresponding to this, the holder has receptacles that are assigned to the tabs and that are spaced apart from one another, into which the tabs of the collimator plates engage. This configuration ensures highly accurate positioning of even long collimator plates.