Free X duplication chromosomes of Drosophila melanogaster were synthesized by X-ray irradiating the In(1)sc^L8L sc^8R chromosome which has a deletion in the distatl half of hA and the proximal half of hB of the centromeric heterochromatin. Fifty-nine duplications have been isolated and cytogenetically analyzed. They all carry wild-type allele of the yellow gene, y⁺, which should come from the distal tip of In(1)sc^L8Lsc ^8R. They appear to be telocentric and predominantly heterochromatic. Majority of the duplications, especially in the classes MEDIUM and LARGE, can pair with XYL.YS in the male meiosis, indicating that they carry male meiotic pairing site(s) that is known to be located exclusively in the X heterochromatin. Complementation test in the males, Df(1)svr, v/Dp, y⁺, demonstrates that most of the duplications in the classes MEDIUM and LARGE carry euchromatin enough to cover the deletion. The portion of the euchromatin should be of the very proximal region close to the irradiated X chromosome centromere.