Tensile testing of gel-spun hot-drawn ultra-high molecular weight polyethylene (UHMWPE) fibres reveal a ductile-brittle transition temperature. Ductile fracture above the transition temperature is believed to be initiated by a stress-induced orthorhombic-hexagonal phase transition, whereas at lower temperatures brittle fracture occurs in the orthorhombic phase. SEM micrographs of the fracture surfaces of UHMWPE fibres fractured below the transition temperature show fibrillar fracture and cannot be distinguished from fibres fractured in a ductile way above the transition temperature.