The rates of fatigue crack propagation in several heat-treated steels were determined for several alloy steels heat-treated to yield strengths in the range 70,000 to 125,000 psi. For one steel crack growth rates were determined for both a thick notch bend specimen and for thin notched plate specimens. It is observed that the concepts of Griffith-Irwin fracture mechanics successfully correlate crack growth rates in the various geometry and thicknesses employed. However, the specific relationship is dependent on yield strength level. Microscopic examination suggests in some instances that inclusion segregates act effectively to locally accelerate crack growth rates.