Fiddler crabs (Uca pugilator Bosc; Crustacea, Decapoda) feed and court in open, flat spaces. Their visual system has several putative adaptations to their flat habitat, including the tendency to keep the vertical axis of their eyes perpendicular to the plane of the substratum. It is hypothesized that one of the functions of this behavior is to distinguish accurately predators from conspecifics by aligning the region of the eye sensitive to predators with the region in space in which predators occur. To test this hypothesis, a crab was placed in a glass dish, and a moving stimulus was presented between 20 degrees above and below eye level (horizontal). Stimuli below the crab's horizon hardly ever evoked escape responses, while identical stimuli above the horizon produced escape responses whose frequency varied with the angular size, not the absolute size, of the stimulus. Experiments with artificial horizons showed that it is the position of the stimulus relative to the eye that is important, rather than its position relative to external cues such as the visible horizon, the vertical light gradient or the gravitational horizon. It is concluded that a crab responds to stimuli according to their position relative to its retinal equator. This conclusion is discussed in light of the crabs' natural behavior with respect to supra-horizontal stimuli.