The results of pressurized tube experiments designed to investigate the creep behavior of two precipitation hardened alloys (M813 and PE16) of the Fe-Cr-Ni system are presented. Data are reported covering temperatures from 400°C to 750°C, for accumulated fluences to 16 × 10²² n/cm², E > 0.1 MeV. This corresponds to five years of testing in EBR-II. Both alloys show greater swelling and creep resistance than 316 CW. A peak in the creep temperature dependence of M813 and PE16 was found that corresponds to the peak swelling temperature. The in-reactor creep resistance is rapidly lost for irradiation temperatures greater than 675°C in both alloys. The results support micromechanisms of irradiation creep which are theoretically linked to the swelling phenomena.