AbstractMales ofUrosimulium stefaniiCont., (1963) from various populations in Sardinia, differ from females by a long asynaptic region in one pair of salivary gland chromosomes. This asynaptic tract does not seem to be due to inversions but rather to large structural changes. In meioses of the male, a pair of chromosomes does not show interstitial chiasmata. It is suggested that this pair of chromosomes represents the sex chromosome pair. The evolutionary process has been discussed.