AbstractAn investigation into the number of chromosomes in thirty‐seven species of Aphidiae belonging to twenty‐seven geaera has shown that there is but one sex chromosome in the male cells of all but one species of Aphididae. That exceptional species is Euceraphis betulae Koch.The number of chromosomes and body characters are so closely correlated that we can safely judge the evolutional scale of any aphid by its number of chromosomes.In Aphididae the least number of chromosomes seems to be the most primitive.The number of chromosomes varies with the genera. The highest number is 1811+ X = 37 chromosomes, the lowest being 211+ X = 5 (diploid in male).The increase in the number of chromosomes seems to have been brought about by transverse divisions of the primitive chromosomes.The genus Tuberolachnus with 311+ X = 4 elements (♂) represents altogether different chromosomes characters from the genus Pterochlorus with 8 elements.In the genera Periphyllus and Calaphis there are species with a smallm‐element which is bivalent.