The timing of tectonic events is necessary for the knowledge of the evolution of mountain ranges. The dating of shear zones is especially useful to constrain the successive thermo-tectonic events which affect terranes. The development of mylonitic zones of regional extent is one of the most striking structural features of the Pyrenees. In the Northern Pyrenean Massif of St. Barthe´le´my, shear zones which deform the Variscan high-grade metamorphic series were recently studied. In order to investigate the behavior of the 40Ar- 39Ar isotopic system during a significant shearing episode and to determine the age of these shear zones, 40Ar- 39Ar isotopic analyses were carried out on the sheared gneisses of the St. Barthe´le´my Massif.The biotites from sheared migmatites and granulites of the gneiss core all yield well-defined plateau ages which are bracketed between 110 and 100 Ma. The muscovites of the same samples yield low-temperature ages which agree with these values. We conclude that a major mylonitic event occurred in the St. Barthe´le´my Massif at ∼110-100 Ma. The motion of the Main Mylonitic Band lasted up to — or started again from— ∼90 Ma, as attested by the biotite age spectrum of a mylonite. The motion of these shear zones is contemporaneous with the crustal thinning and the occurrence of a high thermal gradient during Mid-Cretaceous time in the North Pyrenean Zone. Later disturbance is recorded by some of the analyzed minerals at ∼55–60 Ma. Unsheared gneisses from the Trois Seigneurs Massif close to the studied area give an older age of 240 Ma, which is intermediate between Variscan and Pyrenean ages. The lack of Hercynian ages in the mylonites of the St. Barthe´le´my Massif is not conflicting with possible previous Variscan mylonitic events.