In the field vole, Microtus agrestis, most of the constitutive heterochromatin is localized in the giant chromosomes. A detailed examination of a large number of adult cell types reveals that this chromatin is actually present as a heterochromatic fiber in all interphase nuclei. Depending upon the cell types, however, the fiber shows varying degrees of condensation and folding ranging from a very long and extended fiber to a large compact chromocenter. The number of cell types with giant chromocenters was less commonly observed than those with extended fibers. This explains why some cells were previously thought to be devoid of heterochromatin.—The results of this investigation strongly indicate that constitutive heterochromatin represents a unique nuclear entity.