A positive carbon isotope (δ13C) excursion has been recognized in upper Kinderhookian and early Osagean carbonates in three sections in southeast Idaho and Nevada. The oldest δ13C peak (+7‰) is dated to the isosticha conodont zone, and a younger peak occurs in the typicus Zone. The shifts are recorded in a range of carbonate lithofacies representing various water depths along the shelf. Lithofacies sampled for δ13C and δ18O at the Samaria Mountain section in southeast Idaho record the shallowest-water conditions, indicated by cross- bedded skeletal and peloidal grainstones. The deepest water conditions are present in the Pahranagat Range section in eastern Nevada, which consists mainly of bioturbated lime mudstone and skeletal wackestone. The δ13C values from these widely separated sedimentary basins show a consistent trend that correlates with Early Mississippian curves generated from brachiopod calcite in western Europe and the Midcontinent of North America, as well as dolomites in Utah and Wyoming. δ18O values become more positive up section, generally paralleling the positive trend in δ13C during the late Kinderhookian. No subaerial exposure surfaces are recognized in the sections examined in southeast Idaho and Nevada, and at least the δ13C trends are interpreted as primary seawater fluctuations. Sea-level changes occurred near the beginning of the late Kinderhookian δ13C shift (early to middle parts of the isosticha Zone) and within the peak of the δ13C excursion (Kinderhookian-Osagean boundary), although tectonic changes associated with the Antler orogeny have likely modified the eustatic signature.