The chromosome numbers of 136 species of the Spanish caraboid fauna were studied. The most frequent karyotypes are 2n=37 (54 species) and 2n=24 (23 species), and the chromosome number ranges from 2n=21 to 2n=69, of which 2n=69 is the highest diploid number hitherto found among the Coleoptera. It is proposed that 2n=37 is the ancestral karyotype of the division Caraboidea and the suborder Adephaga as opposed to that of the suborder Polyphaga, 2n=20. Karyotypic evolution has led to increases and decreases of this number, both tendencies having taken place in four genera. Species of ten genera show a neo-XY bivalent due to an X-autosome fusion. The thirty-three chromosome numbers of Caraboidea reveal that these Coleoptera have a remarkable karyotypical heterogeneity.