The variation of the postirradiation creep-rupture properties with irradiation temperature has been evaluated for air- and vacuum-melted Hastelloy-N. The air-melted material was high in silicon and formed a stable carbide of the M6C type. The properties of this material were not dependent upon the irradiation temperature over the range studied. The vacuum-melted alloys formed a M2C-type carbide whose size and morphology depended markedly upon the irradiation temperature. When the carbides were finely dispersed by irradiation at about 650°C, the postirradiation properties were equivalent to those of the air-melted material. Irradiation at about 760°C resulted in coarser dispersions of the M2C carbide and inferior postirradiation properties.