The Great Glen Fault Zone is a major, subvertical, reactivated fault within the Scottish Caledonides. Post-Caledonian dextral movements of a few tens of kilometres have been demonstrated previously from the displacement of geological markers. Late Caledonian sinistral displacement has been generally inferred, but only by comparison with apparently related structures in the Central and Northern Highlands. Critical outcrops of low-middle greenschist-facies phyllonites, mylonites and cataclasites, particularly at Torcastle near Fort William, provide new evidence for the nature and sense of early displacements along the Great Glen Fault Zone. Shallow to moderately plunging mineral lineations and shear-sense indicators suggest sinistral displacements that are thought to be late Caledonian (c. 425-390 Ma) based on regional considerations. Although the Great Glen Fault Zone may at present separate contrasting basement blocks, there is no evidence that the Caledonian displacements were associated with the reactivation of an older Proterozoic structure.