As part of a program to provide a data base on the bainitic alloy Fe-2¼Cr-1Mo for fusion energy applications, microstructural examinations are reported for nine specimen conditions for 2¼Cr-1Mo steel that had been irradiated by fast neutrons over the temperature range 390 to 510°C. Void swelling is found following irradiation at 400 to 480°C. Concurrently dislocation structure and precipitation developed. Peak void swelling, void density, dislocation density, and precipitate number density formed at the lowest temperature, approximately 400°C, whereas mean void size and mean precipitate size increased with increasing irradiation temperature. The examination results are used to provide interpretation of in-reactor creep, density change, and post irradiation tensile behavior.