The first occurrence of Tetragraptus approximatus approximatus is now taken to mark the base of the second (＇＇Arenig＇＇) series of the Ordovician System. We propose that the Cow Head Peninsula, western Newfoundland, be adopted for Global Stratotype Section and Point (GSSP), with the boundary placed at the base of the T. approximatus Zone, 2.9 m above the base of the Factory Cove Member, Shallow Bay Formation of the Cow Head Group. The well-preserved graptolite fauna occurs in association with abundant conodonts, demonstrating this level to correspond closely with the base of the Prionodus elegans conodont Zone. Immediately underlying strata yield bithecate graptoloid assemblages, generally taken as indicating the latest part of the previous (＇＇Tremadoc＇＇) series. Neighbouring sections also yield typically Tremadoc deep-water trilobites from this level, permitting correlation between the three fossil groups. Other fossils, including acritarchs, are also present in the Cow Head Group sections, which are structurally simple, of low thermal maturity (CAI 1.5 or less) and suitable for chemo- and magnetostratigraphic studies.