The analysis of chromosome number and sex-determining system in fourteen species of chrysomelids belonging to four tribes of Chrysomelinae has shown a modal number of 2n= 24 chromosomes and Xypsex-system.Gastrophysa polygoni, Mesoplatys cincta(Phaedonini),Gonioctena nivosa, G. linnaeana, G. quinquepunctata(Phyllodectini),Chrysolina coerulans, C. bicolor, Oreina ganglbaueri (Chrysolinini),Paropsis reticulata, Paropsisternd octosignata, P. beataandChrysophtharta variicollis(Paropsini) have karyotypes of 24 chromosomes whileCyrtonus punctipennisand C. plumbeus (Phaedonini) have 28 chromosomes. In spite that the modal number of the Chrysomelinae seems to be 2n= 24, the most primitive number for the subfamily is probably 2n= 20, in agreement with the primitive number of Coleoptera Polyphaga. This number is only encountered in the ancestral genusTimarcha, being all the remaining numbers derived from the primitive one.