Seven males of the stick insect B. rossius (out of the twenty collected on the Isola del Giglio in July 1974) have shown a chromosomal rearrangement of a definite kind in all the spermatogonia and spermatocytes I which could be analysed in spite of extensive stickiness. Five of them had a pericentric inversion affecting one of the medium sized autosomes (4 heterozygotes and 1 homozygote); this chromosomal rearrangement appears to be the same in the 5 specimens. For one of the heterozygotes it has been possible to investigate two spermatocytes I at metaphase and a corresponding heteromorphic bivalent has been observed in both. The remaining two males have been shown to posses in their spermatocytes I a small bivalent with partners of unequal size; it has not been possible to decide whether this structural change is the same in the two specimens; for one of them it is likely that the condition arises from a translocation of a part of the large metacentric chromosome to an autosome of a small acrocentric pair. Furthermore several instances of structural and numerical changes have been observed in single cells. All these findings are in agreement with a widespread occurrence of chromosomal breakages in B. rossius as has been already found in other stick insect species. Stickiness could also be due not to technical faults but to cytological features of the Cheleutoptera.