Rotating bending fatigue measurements are reported for an austempered ductile iron containing 3.5wt% C, 2.6wt% Si, 0.48wt% Cu, 0.96wt% Ni, 0.27wt% Mo, and 0.25wt% Mn. The iron was austenitized at 870, 900 and 950 degrees C and then austempered at 370 and 400 degrees C for times between 30 and 240 min to obtain various austempered microstructures. The correlation between fatigue strength and austempered microstructure represented by the para meter X gamma C gamma, where X gamma is the amount of high C austenite and C gamma its C content is examined. It is shown that fatigue strength increases as X gamma C gamma increases. The highest fatigue strength is obtained with an ausferrite structure; the presence of martensite and/or carbide in the structure reduces the fatigue strength. Lower austenitizing temperatures increase the fatigue strength.