The genomic and intrachromosomal control of condensation inhibition of heterochromatic regions of mitotic Drosophila chromosomes was studied. It is shown that decondensation is not a local property of the chromosomal regions. In particular the frequency and sometimes the pattern of decondensation is changed with translocations, inversions or loss of chromosomes. In a variety of deletions and inversions of X-chromosomal heterochromatin, which is usually decondensed, the phenomenon is absent or its pattern changed. A material effect is observed in interspecies hybrids between D. melanogaster and D. simulans: paternal chromosomes are neither more condensed or more decondensed under the influence of a maternal genome.