Technical Abstract: Temperate grasslands are vast terrestrial ecosystems that may be an important component of the global carbon (C) cycle; however, annual C flux data for these grasslands are limited. The Bowen ratio/energy balance (BREB) technique was used to measure CO2 fluxes over a grazed mixed prairie and a seeded western wheatgrass [Pascopyrum smithii (Rybd) Love] site at Mandan, ND from 24 April to 26 October in 1996, 1997, and 1998. Above- ground biomass and leaf area index (LAI) were measured about every 21 days throughout the season. Root biomass and soil organic C and N content were determined to 110-cm depth in selected increments about mid-July each year. Peak above-ground biomass and LAI coincided with peak fluxes and occurred between mid-July to early-August. Biomass averaged 1227 and 1726 kg/ha and LAI 0.44 and 0.59, for prairie and western wheatgrass, respectively. Average CO2 flux for the growing season was 279 g CO2/m2 for prairie and 218 g CO2/m2 for western wheatgrass. Fluxes during the dormant season estimated from soil temperatures averaged -258 g CO2/m2. Annual fluxes, calculated from growing season and dormant season fluxes, averaged 21 g CO2/m2/y for prairie and -40 g CO2/m2/y for western wheatgrass. However, BREB measurements of dormant season flux for the prairie in 1999-2000 was only -151 g CO2/m2 which gave an annual flux of 128 g CO2/m2/y, considerably higher that the annual flux calculated from soil temperature. These results suggest Northern Great Plains mixed prairie grasslands can either be a sink for atmospheric CO2 or near equilibrium, depending on the magnitude of the dormant season flux.