Edge-cracked test specimens of two ferritic steels were subjected to the loading cycle σ^m ± σ^a at 300 C. The results showed that whether a crack grows or remains dormant can be predicted from the value of ΔK, the range of stress intensity factor during the fatigue cycle. The threshold value ΔK^c necessary for crack growth was slightly greater than that of similar steels at room temperature and depended on the ratio σ^m/σ^a. Crack growth data at low rates of crack growth were deduced from an integrated version of the crack growth equation da/dN = B(ΔK)^m, where B and m are material constants. Crack growth rates at 300 C were similar to those at room temperature. The minimum possible rate of fatigue crack growth appeared to be somewhat greater than one lattice spacing/cycle. The methods used have the advantage that no crack growth monitoring instrumentation is required.