Data from FIFE (the First International Satellite Land Surface Climatology Project (ISLSCP) Field experiment) were used in an atmospheric boundary layer bulk similarity formulation to derive values of the friction velocity, u* (i.e., the shear stress), from surface geostrophic wind measurements under unstable conditions. Several stability-dependent forms for the similarity functions B (corresponding to the x direction geostrophic wind, ug) and Bw (corresponding to the scalar geostrophic wind speed, G) were tested. Formulations to calculated u* from G were more successful than those which used only ug. However, the dependence of B, Bw, or the geostrophic drag coefficient, CG=u*/G, on either stability or baroclinicity was observed to be relatively weak. For FIFE, a constant value of Bw=1.99 agreed with the reference u* values almost as well as the best stability-dependent formulations. A constant value of CG=0.0883, used with measured values of G, gave an unbiased estimate of u*, on average, with a correlation r=0.80.