Graptolite zones of early and late Tremadoc ages are described from the Elmer Creek Formation in the Selwyn Basin and from the Duo Lake Formation in the Misty Creek Embayment. In ascending order, these are: Staurograptus dichotomus, Anisograptus matanensis, Adelograptus cf. A. tenellus and Aorograptus victoriae Biozones. The uppermost zone embraces the Adelograptus antiquus Biozone, Kiaerograptus? pritchardi Biozone, and the Paradelograptus kinnegraptoides Biozone in the Richardson Trough. Twenty five species are recorded and fourteen species are described, including the first record of the Rhabdinopora fauna in the early Tremadoc of northwest Canada and Kiaerograptus magnus Williams and Stevens and Paratemnograptus isolatus Williams and Stevens in the late Tremadoc. When modifications to Cooper’s (1999) ecostratigraphic model are made based on data from Yukon and Northwest Territories, the upper slope biotope and lower slope – ocean-floor biotope become so similar that there is little basis for recognising both biotopes.