Hundreds of trilobite impressions were collected from graptolite-bearing shale in the type locality of the Upper Ordovician Saturday Mountain Formation of central Idaho. Ampyxina salmoni, n.sp., Primaspis sp. and Toernquisitia? idahoensis, n.sp., are described from this trilobite faunule. Ampyxina and Primaspis were previously unrecorded from the Western U.S. and Toernquistia was unknown from North America. On the basis of the trilobites and associated graptolites the shale is Trentonian and seems correlative with Elles & Wood graptolite zones 11 and 12 in Great Britain. This fossiliferous shale is most likely part of the predominantly carbonate Saturday Mountain succession, thus extending the age of the formation to include Trentonian.