Fatigue-crack propagation studies were carried out on some ultrahigh-strength steels in controlled environments using the fracture-mechanics approach. The steels investigated include a medium-carbon low-alloy ultrahigh-strength steel tempered to two strength and toughness levels, a 250-grade 18Ni maraging steel, and a 300-grade 18Ni steel. The results indicate that, in the inert environment, the rates of fatigue-crack propagation in these steels are about the same and show no obvious correlation with other mechanical properties of the steels. The sensitivity of the rates of fatigue-crack propagation to the presence of moisture in the test environment shows rough correlation with plane-strain fracture toughness for steels of similar composition. The role of water vapor in promoting fatigue-crack propagation is considered. The results are also discussed in terms of existing theories and metallographic observations.