Mode I interlaminar fracture toughness was determined for a high-temperature thermoplastic matrix, intermediate modulus graphite fiber composite laminate. Interlaminar fracture toughness and fracture energy were determined for a unidirectional layup, continuous-fiber reinforced thermoplastic composite by using a nonconventional specimen. A short bar hybrid test specimen (HTS) was developed and used in this investigation to determine the effect of moisture on mode I delamination fracture toughness and critical strain energy release rate. Exposure of the thermoplastic composite laminate in a water bath for over 400 h had an insignificant effect on delamination fracture toughness. Moreover, a fractographic examination did not reveal effects of absorbed moisture on fracture surface morphology and failure processes. Fracture was characterized primarily by matrix decohesion and fiber pullout. There was, however, clear evidence of fracture mechanisms that contributed significantly to mode I fracture energy of the composite. Crack bridging by isolated, single fibers and fiber bundles was prevalent, particularly after a considerable degree of crack extension in the laminate. Translaminar, secondary microcracking was observed in the vicinity of the primary crack tip.