Organization of chromosomes was analyzed in ovarian nurse cells in 12 species of the virilis group of Drosophila. A diffuse chromocenter containing a clearly detectable block of heterochromatin was shown to be characteristic of the phylogenetically ancestral species D. virilis and D. kanekoi. Similar organization of chromosomes was observed in two daughter species, D. lummei and D. falvomontana. The chromocenter disappeared either completely or partially in the phylads montana and virilis, respectively. These species exhibited separate chromosomes attached to a nuclear envelope and dispersed within the nucleus. Detected alterations in chromosome architecture confirm evolutionary trends previously revealed in species of the melanogaster subgroup.