Measurements using compact tension specimens machined from compression and injection-molded plaques of polyoxymethylene show a systematic decrease in the plane-strain critical stress intensity K-IC for crack initiation with crystallization temperature and with molecular weight, but little change in the yield stress in plane strain compression tests. The variation in K-IC with crystallization temperature has been accounted for in terms of the effective entanglement density in the damage zone ahead of the crack tip. The entanglement density is argued to decrease with increasing temperature owing to partial disentanglement of the polymer chains during lamellar folding.