The surface sensible heat flux, H, and the latent heat flux, LE, were determined under unstable conditions for the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) site in eastern Kansas. NOAA 9 and 11 AVHRR radiative surface temperatures, corrected for atmospheric effects using the split window technique, were used for the surface temperature in a bulk atmospheric boundary layer similarity formulation for H. Mean vertically integrated mixed layer temperatures, determined from radiosonde data, were also required in this technique. However, unlike previous applications, the present approach avoids the need for direct wind speed measurements by using instead the geostrophic wind speed, VG, as derived from the routine measurements of the surface pressure field. LE was then determined as the residual term of the surface energy balance, using H from the bulk similarity formulation. Under the unstable conditions considered, results for both H and LE were comparable to those found in previous work by means of measured wind speeds with the same FIFE data. It is shown that for slight to moderate instabilities a 1% error in the friction velocity generally produces less than 1% error in H. ¿ American Geophysical Union 1995