The effects of finely dispersed fillers on the fatigue performance of asphalt binders and asphalt concrete mixes at relatively low temperatures are examined. A series of model binder systems containing glass spheres with narrow particle size distributions were used to study the effect of filler particle size on the fatigue performance of the asphalt mastic. Two mastic systems containing ground limestone fillers, which possessed significantly different gradations, also were tested. Fatigue performance was evaluated by applying a constant torsional strain to each specimen in a dynamic rheometer at 10 deg C and 40 Hz. Testing at various strain levels allowed the relationship between fatigue life and strain to be determined for the different systems. The results indicate that as the particle size of the filler decreases, the fatigue life of the asphalt mastic increases. This observation is a direct result of the mode of fatigue failure in the asphalt mastics and is in agreement with Evans's theory on crack pinning for failure in filled brittle solids. Constant stress asphalt concrete fatigue tests on both dense- and gap-graded systems prepared with the two different ground limestone fillers show that the particle size does not significantly affect the fatigue life of the mixes. These results also confirm that crack pinning is the major mechanism responsible for improved fatigue performance.