Crystalline nylon 66 was modified by blending with both an amorphous nylon and a rubber-modified amorphous nylon. The ternary blends exhibit a 50-100-fold decrease in fatigue crack propagation rates, even at rubber concentrations of only 1 or 2%. These same blends do not necessarily exhibit improved impact strength and the examination of a variety of alloys and blends demonstrates that fatigue and impact fracture mechanisms are distinctly different. The fracture surface morphologies indicate that the basic fatigue fracture mechanism of craze coalescence for nylon 66 is not changed by alloying. However, the presence of the rubbery phase leads to cavitation and ductile drawing that retard the craze breakdown and coalescence processes without evidence of crack tip blunting. Surprisingly, the addition of rubber-modified nylon 66 to a nylon 66 matrix does not impart as great an improvement in fatigue resistance as does the miscible amorphous nylon. Also, alloys having improved impact strength are observed to exhibit inferior fatigue resistance. These results demonstrate that the excellent fatigue resistance of crystalline polymers can be improved even further by judicious selection of alloying ingredients optimized specifically for fatigue fracture.