This study examined the interaction between grouping information and expertise in a simple enumeration task. In two experiments, participants made rapid judgements about the number of items present in a visual display. Within each display, items were grouped into a canonical representation (e.g., triangle, square, and pentagon) or were arranged linearly. In both experiments, grouping information facilitated enumeration performance, replicating previous findings in the literature. In Experiment 2, the facilitative effect of grouping information was found to be greater for Air Traffic Controllers (ATCs) than for matched novices, though they were no better than novices on linear arrays. This may be because linear, like canonical arrays, hold unique numerosity information, but only when they contain the minimum number of points necessary to define a line (i.e., 2). So ATCs' performance on linear arrays containing more than two items does not benefit from a facilitative effect of grouping information. That their experience of being ATCs, in terms of years served, was shown to account for the expertise effect suggests that such visuospatial expertise is acquired through frequent exposure to spatial arrays.