Orientation bandwidths of cyclopean channels were estimated using a notched noise technique. Observers were presented with random dot stereograms depicting a horizontal or vertical target sinusoidal depth modulation and a mask consisting of sinusoidal depth modulations whose orientations flanked that of the target. Masking reduced as the orientation difference between signal and mask increased. The orientation bandwidth of the masking effect was similar to that found for stimuli defined by luminance contrast, and showed no systematic difference for horizontal and vertical targets. These results suggest that the elongated summation found by Tyler, C. W., and Kontsevich, L. L. (2001). Stereoprocessing of cyclopean depth images: Horizontally elongated summation fields. Vision Research, 41, 2235-2243, for horizontal stimuli occurs after a processing non-linearity.