New field collections from sections in western Utah and southeastern Idaho permit the development of a high resolution trilobite biostratigraphy for the northern Laurentian Lower Ordovician (upper Tremadocian) Stairsian Stage similar to that proposed previously for the overlying Tulean and Blackhillsian stages. Four zones recognised previously are replaced with 11 formally proposed zones, most of which are new in concept. The new zonal scheme in ascending order includes the ’Paraplethopeltis genacurva’ Zone (replaces ’Paraplethopeltis Zone’/’Zone C’), ’Paraplethopeltis helli’ Zone (new), ’Hystricurus zanderi’ Zone (new), ’Rossaspis leboni’ Zone (new), Unnamed Zone 1 (new), ’Bearriverops loganensis’ Zone (new; contains much of the diversity previously assigned to ’Tesselacauda Zone’/’Zone E’), ’Bearriverops deltaensis’ Zone (new), ’Bearriverops alsacharovi’ Zone (new), ’Pseudoclelandia weymouthae’ Zone (new), ’Pseudoclelandia cornupsittaca’ Zone (new) and ’Pseudohystricurus obesus’ Zone (new). The latter two zones were previously lumped as the ’Rossaspis superciliosa Zone’/’Zone F’. Four of the name bearers of the new zones, ’Paraplethopeltis helli’ sp. nov., ’Hystricurus zanderi’ sp. nov., ’Rossaspis leboni’ sp. nov. and ’Pseudoclelandia weymouthae’ sp. nov., are formally described.