The properties of constitutive heterochromatin so far described, perhaps with the exception of the possibility of being C-banded, are not all common to all the species examined. The use of staining techniques orin vivotreatments makes it possible to demonstrate the existence of a marked heterogeneity of fractions of heterochromatin sometimes located on the same chromosome or even belonging to the same heterochromatin block. Moreover, the analysis of the DNA sequences located in the heterochromatin demonstrates the existence of a marked molecular heterogeneity. The subject is treated in greater detail with regard to the human karyotype.