In-reactor creep data to a peak fluence of 5.7 × 10²² neutrons (n)/cm² (E > 0.1 MeV) for irradiation temperatures between 380 and 570°C have been obtained for the ferritic alloys HT9, 9Cr-1Mo, and 2¼Cr 1Mo. Also, in-reactor creep data for HT9 has been obtained at 650°C at a fluence of 2.9 × 10²² n/cm² (E > 0.1 MeV). For irradiation temperatures less than 520°C, the creep strains for all ferritic alloys are small and less than predicted for 20% cold-worked Type 316 stainless steel. At 570°C, thermal creep mechanisms become important and 2¼Cr-1Mo loses its creep resistance. HT9 loses its creep resistance at 650°C and exhibits large diametral strains (>24%).