The discovery and dating of a volcanic ash bed within the upper Phosphoria Formation in SE Idaho, USA, is reported. The ash occurs 11 m below the top of the phosphatic Meade Peak Member and yielded a 206Pb/238U date of 260.57 ± 0.07 / 0.14 / 0.31 Ma, i.e. latest Capitanian, Guadalupian. The stratigraphic position of this ash near the top of the Meade Peak phosphatic Member of Phosphoria Formation indicates plausible completeness of the sedimentation within the Guadalupian–Lopingian and probably at the Permo-Triassic (P-T) transitions. The new radiometric age reveals that the regional biostratigraphy and palaeontology of Phosphoria and Park City formations requires serious reconsideration, particularly in cool water conodonts, bryozoans and brachiopods. The new age proposes that the Guadalupian–Lopingian boundary (GLB) coincides with the Meade Peak – Rex contact and consequently with the end-Guadalupian extinction event. The lack of a major unconformity at the P-T transition suggests that the effects of the Sonoma orogeny were not as extensive as has been assumed.