Species of the Chironomus plumosus group display significant differences in their amount of centromeric heterochromatin. A tandem-repetitive satellite-like DNA has been isolated from C. plumosus. This DNA accounts for a major part of the centromeric heterochromatin. The DNA element has a Sau3AI restriction site ("Sau elements") and a monomer length of 165 or 166 bp. It is A-T rich (73%) and reveals a moderate DNA curvature, as shown by gel migration and computer analysis. The chromosomal localization and genomic organization of Sau elements were studied in 24 Chironomus species by in situ hybridization and (or) Southern analysis. The DNA is predominantly located in the centromeric regions of nine species, six from the plumosus group and three others. In some cases, inter- and intra-specific differences in the size of the centromeric heterochromatin seem to correlate with the strength of Sau element hybridization signals. Few species contain Sau repeats outside of the centromeres (C. borokensis, C. plumosus). Additionally, Sau elements are revealed to be present in ectopic threads between centromeres, and in B chromosomes found in C. borokensis and C. annularius.Key words: satellite DNA, curved DNA, centromere, heterochromatin, Chironomus.