The effect of resin flexibility on the fracture surface characteristics of a series of polyester resins has been investigated by scanning electron microscopy. Fracture surfaces were obtained from fracture toughness specimens, un-notched tensile specimens and notched specimens cycled over a constant range of stress intensity factor. Characteristic surface markings have been identified with regions of stable crack growth, slow unstable crack growth and fast unstable crack growth. The three regions were only present on the surfaces of failed fracture toughness specimens. The extent of each region is a function of the resin flexibility. The absence of a given region on the fracture surfaces of other specimens is a function of the loading conditions.