Measurements of net CO2 flux density (¿mol m-2 s-1) from a high elevation, sagebrush steppe community in southeastern Wyoming (USA) were compared quantitatively among four different instrument systems (leaf cuvette and 1 m2 community chamber for gas exchange measurements; tower and aircraft eddy covariance systems) by minimizing spatial and temporal variability. Ground-based flux measurements were recorded at an intensive site located near the midpoint and directly beneath an approximate 20-km flight transect. A high degree of homogeneity in plant species composition, density, cover, and the amount of leaf area per unit ground area, as well as little topographic variability, was measured at the intensive site and along the flight transect. Flux measurements were compared on days with relatively high and low soil moisture availability (predawn plant water potentials >-0.8 MPa and <-2.0 MPa, respectively). Same-day, mean flux values between the four measurement systems (4.0--4.6 ¿mol m-2 s-1) over identical time intervals (0900--1100 hours) varied by a maximum of ¿9% (maximum range 23%). Ground-level measurements taken within ¿1 day of flight measurements, varied by a minimum of ¿7% (25% maximum range) of aircraft values. This difference increased curvilinearly to a maximum of ¿31% (38% maximum range) for a 2-week separation between flight and ground-based measurements. Thus, under near-ideal conditions of topographic and vegetative homogeneity, temporal heterogeneity in the measurement regime of only a few days resulted in greater disparity in measured CO2 flux density than occurred among the four instrument types.