Parameters derived from eddy correlation measurements of the air-surface exchange rates of H2O, CO2, and O3 over a tallgrass prairie are examined in terms of their relationships with spectral reflectance data remotely sensed from aircraft and satellites during the four 1987 intensive field campaigns of the First ISLSCP Field Experiment (FIFE). The surface conductances gc were strongly modulated by photosynthetically active radiation received at the surface (K↓par) when the grass was green and well watered; mesophyll resistances were large for CO2 but negligible for H2O and O3. Seasonal changes of the surface conductances for a given level of K↓par were well correlated with changes in the simple ratio (SR) of canopy reflectances in the red and near-infrared wave bands. A linear relationship in the form of gc=G0+β(SR--SR0), where SR0≈2 represents the simple ratio for senescent grass and G0 represents the corresponding nonstomatal conductance, fits the data fairly well and thus supports the results of modeling studies by Sellers (1985, 1987). In this relationship, G0 had values of near zero for H2O and CO2 and about 0.4 cm s-1 for O3. The slope β for large irradiation was about 0.08 cm s-1 for H2O and O3 and 0.03 cm s-1 for CO2. The inferred nonstomatal conductance had a small seasonal variability in comparison to the total canopy conductance. ¿ American Geophysical Union 1992