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Aridity is expected to affect ant diversity by limiting the availability of resources and causing physiological distress to the ant populations. Studies conducted so far on such relationships are few and results are inconsistent due to confounding factors. Our aim was to survey an area that had an extended rainfall gradient, with minimal temperature and topography variations, to determine whether ant diversity was negatively correlated to aridity. The study was carried out in 11 localities along a 400 km-long transect in the Paraguayan dry Chaco. Mean annual rainfall ranged from 460 to 887 mm. In total, 197 ant species were collected. At the studied regional scale, our results did not confirm that aridity constitutes an environmental filter reducing ant diversity, neither at specific nor at higher-phylogenetic levels. Based on our results and of those extracted from the literature, rainfall seems to have either no influence or a negative one on ant alpha-diversity at both regional and larger geographical scales, except when rainfall is very low (e.g. <200 mm). This pattern contrasts with what is classically observed for a large majority of other taxa. (C) 2010 Elsevier Ltd. All rights reserved.