The monospecific genusBaeticais characterized by extensive amounts of C-heterochromatin, placed on M3, M4and M5chromosomes, as large c-blocks or some intestinal C-bands. The only karyotypic variation founded among the analysed individuals of this genus is a considerable degree of polymorphism for the presence, placement and amount of C-band positive chromatin. These differences seem to be related to the additions of constitutive heterochromatin, which probably constitutes the major mode of karyotypic change occurring in the evolution ofB. ustulata.Significative differences exist between the observed frequencies of the various genotypes and the Hardy-Weinberg expectations, in both M4and M5chromosome pairs.