The effect of He on the primary damage induced by irradiation in ultrahigh-purity (UHP) Fe and Fe(Cr) alloys was investigated by transmission electron microscopy (TEM). Materials were irradiated at room temperature in situ by TEM in a microscope coupled to two ion accelerators, simultaneously providing 500 keV Fe+ and 10 keV He+ ions. Single Fe ion and dual Fe and He ion beam experiments were performed up to a dose of 1 dpa and to a He content of up to 1000 appm. Defects appear in the form of nanometric black dots with sizes between 1 and 5 nm. Defocused images reveal a dense population of sub-nanometric cavities after both single-beam and dual-beam irradiation. In Fe(Cr) alloys, the number densities of visible black dot defects still resolved in TEM are significantly higher after single than after dual-beam irradiation. In UHP Fe, conversely, the presence of He strongly increases the defect number density. The presence of He changes a a(0)< 1 0 0 > dominated defect population to a 1/2a(0)< 1 1 1 > dominated one in all materials, and the more so in UHP Fe. It appears that Cr increases the number of visible defects relative to UHP Fe. The dependence with increasing Cr content is weak, however, showing only a slight decrease in the number densities. The decrease in the density of visible a(0)< 1 0 0 > loops and increase in the visible 1/2a(0)< 1 1 1 > loops in all materials when He is present supports the idea that visible a(0)< 1 0 0 > loops are formed by the interaction between mobile 1/2a(0)(1 1 1) loops, as the latter would be immobilized by He already at sub-microscopic sizes. It is concluded that the primary loop population is dominated by 1/2a(0)< 1 1 1 > loops. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.