The critical stress field intensity factor for crack propagation, KIc, was determined for a large number of glass fiber-reinforced acetal copolymer compositions and for the unfilled resin. The results were interpreted in terms of a model previously proposed for the tensile behavior of these materials. The KIc could be regarded as a linear function of the contribution of the fiber reinforcement to the tensile strength, but was otherwise substantially independent of the amount and length of the fibers and the nature of the fiber finish. From this relationship it was estimated that the inherent flaw size of these materials was of the order of magnitude of the fiber length. The observed variation of KIc with loading rate was also consistent with the model. The notched Izod impact strength of these same materials was shown to be roughly equivalent to GIc, the critical strain energy release rate, or fracture energy per unit area, which can be computed from KIc by the methods of fracture mechanics. The behavior of these crack propagation parameters is consistent with the previous hypothesis that failure is initiated by loss of adhesion between the matrix and those fibers which lie transverse to the applied load.