Two sinusoidal gratings equal in contrast and either equal or different in spatial frequency, orientation and spatial phase were presented dichoptically. At low contrast, the subjects perceived a fused image of the gratings, no matter how different the spatial frequencies or orientations on the gratings were. Dichoptic contrast summation of a pair of gratings was compared with physical contrast summation. The ratio between contrast perceived in dichoptic summation versus physical summation of a pair of gratings depends on the difference in spatial frequency and orientation between the two: it approaches 1 if the spatial frequency of the two gratings differs by more than 1 octave or their orientation by more than 10 degrees, and is otherwise lower than 1. Information about the spatial phase of the two monocular inputs is preserved at the level where dichoptic summation takes place.