The effects of various filler characteristics on the ductility of filled amorphous copolyester, Kodar 6763, have been examined. The five fillers in the study included two calcium terephthalates with different particle-size distributions and three calcium carbonates, also with different particle-size distributions. One of the calcium carbonate fillers had received a surface treatment. The increase in Young’s modulus with increasing filler content was the same for all fillers and was satisfactorily described by Kerner’s equation. The only filler to affect the yield stress was the surface-treated calcium carbonate; in this case, the decrease in yield stress was attributed to cracking and splitting of aggregated particles. A sharp drop in fracture strain was observed with increasing filler content. This ductile-to-quasi-brittle transition occurred when the fracture mode changed from fracture during strain-hardening or neck propagation to fracture during neck formation. The critical filler content of the ductile-to-quasi-brittle transition varied from one filler to another. A simple model qualitatively described the decrease in critical filler content with increasing breadth of the particle-size distribution and, in particular, with increasing volume percent of large particles in the distribution.