Several revisions of the taxonomy of the scolytid genus Dendroctonus have been reported during the last century. These have been based on the external morphology and biology of adults, karyology, and most recently molecular genetics. Using karyological data—chromosomal number and mechanism of sex determination—from 119 Dendroctonus populations representing 16 of the 19 species currently recognized, we determined rate of chromosomal evolution and evolutionary phases of this genus. The karyological formulae vary from 5 AA neo-XY (2n = 12) to 14 AA Xyp (2n = 30). The modal number of chromosomes is 2n = 30, with a mean 2n value of 23.37 and coefficient of variation of 31.67%. These values indicate that Dendroctonus has a high rate of chromosomal diversity and that its evolutionary phase is of regression. Theoretically, the genus should be composed of specialist species with relatively low chromosomal numbers; however, this is not the case. We also used chromosomal evidence to examine the suspected Mexican origin for Dendroctonus. We posed two hypotheses to explain the karyological diversification and current distribution of Dendroctonus species. The first considered that diversification of the karyotype originated from species with 14 AA Xyp during its dispersion southward with its hosts (Pinus spp.) in North America. The second assumed that karyological diversification of Dendroctonus occurred during the dispersion of the genus toward Eurasia and southward in North America and that the increases and decreases in number of chromosomes originated from an ancestral karyotype between 18 and 22 chromosomes. Evidence for each hypothesis is discussed.