An Ameriflux site was established in mid 1996 to study the exchange of CO2 in a native tallgrass prairie of north-central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid-July–early August 1996), moisture-stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid-day net ecosystem CO2 exchange (NEE), under well-watered conditions, reached a maximum magnitude of 1.4 mg CO2 m−2 s−1 (flux toward the surface is positive) during peak growth (mid-July 1997), with green leaf area index of 2.8. In contrast, under moisture-stress conditions in the same growth stage in 1996, mid-day NEE was reduced to near-zero. Average night NEE ranged from near-zero, during winter dormancy, to − 0.50 mg CO2 m−2 s−1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y-intercept, so obtained, can be potentially used as an estimate of night-time CO2 emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO2 m−2 d−1 (8.4 g C m−2 d−1) in mid-July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m−2 y−1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near-zero in 1998.