Thirty-one geographic entities comprising 17 species of Calligrapha were examined cytologically. In the bisexual species rowena, philadelphica, pnirsa, amator, alni, confluens, californica, bidenticola, multipunctata, verrucosa, and pruni, the diploid number of chromosomes was 23 in males and 24 in females. The sex mechanism consisted of a single X chromosome in males and two X chromosomes in females. No Y chromosome was present. During reduction division in testicular tissue 11 bivalents were formed and a single heterochromatic X chromosome lay to one side of the bivalents which showed congression at metaphase I. The basic chromosomal formula 11 + XO can therefore be assigned to this group. The formula is modified in some populations according to the following circumstances.