Abstract: The uniquely dominant Australian ant genus Iridomyrmex has been estimated to contain about 350 species, but a recent morphologically based revision of the genus (Heterick & Shattuck, 2011) recognises only 79 species. This issue has important implications for an understanding of the biogeography of, and speciation processes within, this arid continent. We aim to show that the revision does not adequately document true morphological variation within the genus, and that many of the recognised taxa are multiple and often distantly related species, with clear morphological differentiation between populations that are often sympatric. We illustrate this by documenting morphological and genetic diversity within a taxon described by Heterick & Shattuck (2011) as a morphologically uniform species, I. coeruleus. We show that this "species" represents several clearly differentiated morphotaxa with congruent genetic divergence (up to 12%) based on CO1 analysis. These differences are maintained in sympatry, with three of the taxa recorded from a single locality despite one of them showing very limited morphological and genetic variation throughout its range right across northern Australia. We then discuss published morphometric, genetic and distributional evidence from other Iridomyrmex "species" to show that the revision widely under-reports morphological and genetic differentiation and sympatric associations. We argue that the revision therefore gives a misleading picture of true diversity in such an ecologically dominant genus that should be so informative for the biogeography of a continent.