The high variability of the chromosome number observed in the early embryonic stages of the insect Bacillus rossius and the need for karyological comparisons of different populations have suggested a study of its karyotype. The analysis of mitotic divisions in the spermatogonia and of meiotic metaphases I, have shown that the basic chromosome set of the male is made up of two large metacentrics, three medium sized telocentrics (a pair of autosomes plus a single X chromosome), and of fifteen pairs of small acrocentrics. It has been observed that several numerical and structural variations exist in spermatogonia of many individuals and that chromosome breakages are common. In the spermatocytes, however, a much more uniform condition is found so that the onset of meiosis seems to act as a "sieve" for most, but not all, numerical and structural variations. The relevance of these chromosomal characteristics for both cytological interpopulation comparisons and cytotaxonomy is discussed.