Fractography has been used for the postfailure analysis of a filled thermoplastic polyester. The five fracture modes that were previously defined on the basis of macroscopic stress-strain behavior were distinguished by certain fractographic features. These features were characteristic of the fracture mode and did not depend on filler type or filler content. The Mode A ductile fracture surface consisted of two regions : a pullout region of slower crack growth and a rosette region of faster crack growth. The Mode B ductile fracture surface contained only a ductile pullout texture. The Mode C quasi-brittle fracture surface exhibited secondary fracture features that sometimes included the herringbone pattern. The Mode D quasi-brittle fracture surface consisted of a stress-whitened dimple region and a brittle fracture region. The Mode E fracture surface exhibited primarily the rough texture characteristic of brittle fracture. The failure mechanisms inferred from analysis of the fracture surfaces confirmed a microscopic failure model of the ductile-to-quasi-brittle transition in filled PETG that is based on the strain-hardening strength of the polymer ligaments between debonded filler particles.