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We have measured threshold disparity as a function of the spatial frequency (0.25–20 c/deg) and contrast (0.02–0.75) of sine-wave gratings. In forced-choice trials, subjects indicated whether a target grating had crossed or uncrossed disparity relative to a reference grating. Thresholds were lowest near 3 c/deg and rose in proportion to spatial period at lower frequencies. Above 3 c/deg, there were marked individual differences. Across the range of spatial frequencies, disparity sensitivity and contrast sensitivity were correlated (r = 0.84). Threshold disparity was inversely proportional to the square root of contrast. When the contrast seen by one eye was reduced producing unequal monocular contrasts, threshold disparity rose more than when the contrast seen by the two eyes was reduced by the same amount. Our results have implications for stereo models that use zero crossings, peaks and troughs, or centroids as …