The trilobite fauna of the Attawapiskat Formation of Ontario agrees with a late Llandoverian to early Wenlockian age assignment. This fauna is closely related to late Llandoverian and Wenlockian trilobite faunas from eastern Iowa and eastern Greenland. The numerically dominant trilobites of this fauna (illaenids, scutelluids, cheirurids) represent an association that is typical of Ordovician–Devonian carbonate buildups. These trilobites occur in localized accumulations formed by the entrapment of exuviae in depressions or other similar structures.Perryus severnensis gen. et sp. nov. from the Attawapiskat Formation is described. Its relationships with the Encrinuroides/Cromus lineage and possibly with Frammia are discussed.