Void and dislocation structures in an Fe-25Ni-15Cr alloy were studied following irradiation with 2.8 MeV⁵⁸Ni⁺ ions at temperatures between 600 and 750°C (1112 and 1382°F) to maximum damage levels up to 80 displacements per atom (dpa). Void formation was observed at all the temperatures investigated, with the maximum swelling between 650 and 700 °C (1202 and 1292°F). The swelling versus dose relationships exhibited an incubation dose followed by swelling at a rate that increased with increasing damage level. These data were consistent with previous swelling results for austenitic alloys irradiated with charged particles, which indicate that the swelling should become linear with irradiation dose at higher damage levels. Tangled dislocation networks were observed to form at low doses and to be fairly stable up to the highest damage levels examined. With the assumption of the observed stable dislocation networks, the dose dependence of swelling could be explained by a general form of the chemical rate theory for swelling due to void growth.