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Nuclear DNA contents were determined by flow cytometry for 20 populations of the perennial C-4 shrub Atriplex halimus L. (Chenopodiaceae) originating from the Mediterranean basin and Fuerteventura (Canary Islands). Two populations were also analysed for chromosome number: one (from Ibiza, Spain), with a 2C nuclear DNA content of 2.40 pg, was shown to be diploid (2n = 2x = 18), whilst the other (from Sicily, Italy), with 5.11 pg was tetraploid (2n = 2x = 36). With respect to nuclear DNA content, two groups of populations were detected, diploids with 2.40-2.44 pg and tetraploids with 4.77-5.13 pg. The diploid populations were mainly from the western Mediterranean (Spain and France) and Fuerteventura, whereas tetraploids were generally, but not exclusively, from more arid areas in North Africa and the eastern Mediterranean. In general, the diploid and tetraploid populations corresponded to the subspecies halimus and schweinfurthii, respectively. For certain populations having morphologies intermediate between those considered typical of these two subspecies, nuclear DNA contents showed them to be tetraploid. There was significant variation in nuclear DNA content among the tetraploid populations, with greater values in the more easterly populations. (c) 2005 The Linnean Society of London, Botanical Journal of the Linnean Society.