Minisonde data collected in the Athabasca Oil Sands area from 1975 to 1979 were analysed to determine regional values of roughness length (Zo). A rigorous selection re reduced the working data set to a small fraction of the original size. A least squares technique was used to determine from profiles of wind and temperature locally measured near the 50, 100, and 150 m levels. Mean Zo values calculated with allowance for diabatic and displacement height effects ranged from about 8 m downwind of the Syncrude plant site to about 1 m in the Athabasca River valley. Uncertainties in the estimates were of the same magnitude as the mean values. No differences in Zo were found with wind direction. The large values for Zo were attributed primarily to form drag from terrain features in the area during slightly unstable conditions. The study suggested that, where form drag is important, Zo may be stability dependent. An error analysis using reasonable uncertainties for wind speed, balloon height, and temperature gradient measurements showed that probable errors in the estimate of Zo were comparable to the observed variability in Zo.