In Sicily five Bacillus taxa are found: two bisexuals, B. grandii (2n = 34, female; 2n = 33, male) and B. rossius (2n = 36, female; 2n = 35, male), and three thelytokous, B. atticus (2n = 34), B. whitei (2n = 35), and B. lynceorum (3n = 52). Their cytofluorometric 2C DNA values, in picograms, in order are 5.22 (♀) –4.31 (♂); 4.33 (♀) –3.89 (♂); 4.40; 4.53; and 6.80. These values represent the first flow cytofluorometric data on phasmatodean genome size. The highest value of B. grandii and the lowest one of B. rossius appear to be related to abundance and scarcity of C-heterochromatin, respectively, which is also rather low in B. atticus. The genome size of B. whitei agrees perfectly with its shown hybrid derivation from B. grandii and B. rossius. On the other hand the value of B. lynceorum is significantly lower than the one expected (7.38 pg) on the basis of its formerly suggested hybrid origin from the backcross B. whitei × B. grandii, but it appears much closer to, and not significantly different from, the value of 6.97 pg expected according to the newly suggested double allotriploid origin, mainly based on electrophoretic evidence, from B. atticus, B. rossius, and B. grandii. Finally, as far as Bacillus taxa are concerned, average stick-insect genome size appears to be just lower than that of other orthopteroid insects, and much lower (10 times) than that of the leaf insect Extatosoma tiaratum, obtained through static cytometry on Feulgen-stained DNA squashes.Key words: cytotaxonomy, DNA amount, hybrid speciation, parthenogenesis, stick insects.