The effects of an impact modifier, mean stress level, and test frequency on the fatigue crack propagation rate in a miscible amorphous blend of polycarbonate and a copolyester were investigated. Experiments were performed on a servohydraulic testing machine, in load control, using a sinusoidal waveform. The addition of a small amount of impact modifier improved the fatigue resistance of the blend. Fatigue crack propagation rate, da/dN, at any value of crack tip stress intensity factor range, ΔK, in the impact modified blend, was more than a factor of 20 lower than that in the untoughened blend. Significantly more plastic deformation occurred on the fracture surface of the toughened blend than on that of the untoughened blend. The effect of mean stress level on da/dN was determined by varying the minimum-to-maximum stress ration, R, from 0.1 to 0.5. Plots of log (da/dN) vs. δK showed that the fatigue crack propagation rate was unaffected by R and, consequently, the mean stress level. The effect of frequency on da/dN was determined by varying the frequency from 0.25 to 25 Hz. The fatigue resistance of the untoughened blend was not affected by test frequency, whereas the fatigue resistance of the toughened blend increased with increasing frequency. Hysteretic heating at the crack tip was minimal at the highest frequency. The frequency sensitivity of the toughened blend was explained in terms of creep-assisted crack growth. © 1993 John Wiley & Sons, Inc.