The stick insect Bacillus atticus comprises several populations with different chromosome numbers that are distributed over a large range of the Mediterranean basin. Here we have analyzed the DNA content of nine diploid and three triploid populations by flow-cytometry. The mean genome size of the diploids showed a significant decrease from east to west, ranging from 5.29 ± 0.12 pg for the population from Crete (east) to 4.28 ± 0.10 pg for the population from Sardinia (far west). This longitudinal trend of a decrease in genome size from east to west was also found for the triploid populations (from 6.80 pg for the population in Turkey to 6.08 ± 0.01 pg for the population on the Isle of Rhodes). Differences in DNA content between populations belonging to the same species have been described in animals, but the evolutionary implications of these differences are as yet unclear. What emerges from the present study is a correlation between genome-size variations and geographic distribution. The adaptive nature of genome-size variations in response to environmental changes is discussed, and the class of DNA involved hypothesized.Key words: C value, flow cytometry, genome size trends, intraspecific DNA variation, DNA classes.