Recent cognitive models of numerical abilities have postulated that number processing may in part rely on a representation of quantities where magnitude is organized by spatial proximity, along a "mental number line" extending from left to right. We describe four experiments that examined whether such a spatial representation of number would be affected by the presence of unilateral neglect after right hemisphere damage. When asked to judge whether a single number shown at fixation was smaller or larger than "5", patients with neglect were selectively slower to respond to "4", but when asked to compare numbers to "7" they were selectively slower to respond to "6". This is consistent with a representational deficit for numbers located to the left of a reference point along the mental number line and was not found in other right brain-damage patients without neglect. No effect of represented number position was found in a non-numerical task requiring judgements of the physical size of single digit characters. Finally, when asked to classify numbers as indicating hours earlier or later than six o'clock, neglect patients showed a reverse pattern with slower responses to numbers larger than "6', consistent with a representational deficit for hour numbers located on the left side of an imagined clock-face. Our findings demonstrate that unilateral spatial neglect may produce specific representational deficits in number processing that implicate different spatial representations according to the task demands.