Viking bistatic radar data have been processed using Hagfors' scattering function to obtain estimates of rms surface roughness &sgr;B from ground tracks primarily in Mars' equatorial region. Roughness varies from as little as 0.75¿ rms east of Solis and Sinai Plana to at least 8¿ on the western slope of Arsia Mons; these values are appropriate to effective horizontal scales of about 25 m. Estimated roughness in cratered terrain is generally &sgr;B≲5¿, implying the existence of smoothing processes (possibly aeolian) on scales less than 100 m, which are not apparent in orbital images. There is good agreement between bistatic roughness estimates and earth-based results at ground track intersections near the equator in both cratered terrain and plains. Surface tilt probability density functions p(β) derived from these observations are usually more sharply peaked than the Hagfors funtion for similar values of &sgr;B. Both the observed forms and the Hagfors model are considerably sharper near β=0¿ and fall off more slowly at large tilts than Gaussian forms. Earlier results obtained using the Gaussian model (Simpson and Tyler, 1981) appear to have overestimated surface roughness by 30-40%. We infer that the true surface has more flat segments and more ''edges'' than does a gently undulating surface described by Gaussian statistics.