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In order to quantify the availability of vacant favourable or "safe" microsites, the spatial distribution of plants on a sparsely vegetated dolomitic plateau was studied in central Ellesmere Island (Canada). Vegetation (vascular plants and bryophytes), boulders and polygon margins were mapped to the closest centimetre in a 10 m x 5 m plot and the microtopography of 8.5 m x 5 m of that plot was surveyed at a 10 cm x 10 cm scale. Vascular plant density was low (6.9 plants m(-2)). Individuals of the most frequent species found in the plot totalled 300 for Draba subcapitata Simm., 8 for Saxifraga oppositifolia L. and 27 for Papaver radicatum Rottb.. Many plants grew in flat microsites in proximity to boulders and larger plants of the same species indicating successful, though infrequent, reproduction on the site. Still, the total uncolonized area including microsites around boulders and plants was much larger than the area occupied by plants (total plant cover = 0.16%). Clearly, vacant microsites were abundant on this polar desert plateau. However, the rare occurrence of established plants, in spite of the presence of viable seeds, implies that for an available microsite to become a favourable one, additional conditions must be met, such as moisture availability and higher temperatures. Adequate conditions may be met only infrequently, during favourable years.