Radiolaria from the Lower Silurian of the Cape Phillips Formation, Nunavut, Canada allow the definition of four biostratigraphic assemblages. The Haplotaeniatum assemblage extends from the basal Llandovery to topmost Coronograptus cyphus graptolite Biozone. The base is taken as the first occurrence of Haplotaeniatum nunavutensis; no samples were collected below the Ordovician–Silurian boundary. The top of the assemblage is defined by the first occurrence of Orbiculopylorum. The Orbiculopylorum assemblage is marked by the first occurrence of Orbiculopylorum. The boundary between this assemblage and the overlying Plussatispila assemblage is unclear because of a break in stratigraphy (upper Spirograptus turriculatus through Monograptus crispus graptolite biozones). The last occurrence of Orbiculopylorum defines the top of the assemblage. The base of the Plussatispila assemblage is defined by the last occurrence of Orbiculopylorum. A barren interval (Cyrtograptus insectus to lower Monograptus instrenuus – Cyrtograptus kolobus graptolite biozones) separates the Plussatispila assemblage and subsequent Ceratoikiscum assemblage. The first occurrence of Ceratoikiscum is taken as the boundary between these assemblages. The top of the Ceratoikiscum assemblage is not established, but is taken as the first occurrence of Inanihella tarangulica Group radiolarians. This zonation is comparable to, and a modification from, the global radiolarian biozonation of Noble and Aitchison (2000).