An annealed-and-aged ferritic steel containing fine NbC precipitates and relatively coarse particles of Laves phase was bombarded with 4-MeV iron ions to 100 displacements per atom (dpa) at 400 to 600°C. Much of the damage structure consisted of dislocation tangles that coarsened with increasing temperature. Cavity formation began at 450°C, peaked at about 535°C, and was absent at 600°C. Maximum swelling was only 0.2%. The Laves particles exhibited recoil dissolution and restructuring. Bombardments with Fe + He + D or Fe + He beams at ratios of 10 atomic parts per million (appm) He/dpa and 45 appm D/dpa introduced high concentrations of very small bubbles and comparatively few larger, bias-driven cavities. The latter were initiated at bubble clusters on NbC particles. Maximum swelling of 0.75% occurred at 550°C. Recoil dissolution of the Laves phase was retarded by the gases. Large critical cavity sizes and unbalanced dislocation and cavity sink strengths contribute to low swelling of this alloy.