Abstract: A photomultiplier tube includes an evacuated envelope having therein a photocathode, an anode and an electron multiplier disposed between the cathode and the anode for propagating and concatenating electrons along a path therebetween. The electron multiplier and the anode are supported by a pair of oppositely-disposed support plates. At least one aperture which extends along at least a portion of the electron path is formed in each plate. A pair of focusing shields may be spaced from the exterior surface of each of the support plates. The focusing shields are disposed adjacent to the apertures in the support plates and extend longitudinally along the electron path to provide a transverse focusing field which prevents substantially all of the electrons from impinging on the interior surface of the support plates.