Homozygous mh females are fertile at 18 but at 25 they are sterile; irrespective of the males used in cross, nuclei of developing embryos appear to be X-bearing and haploid. 183/200 eggs developed to blastoderm; gastrulation was abnormal; 22 embryos gave evidence of segmentation and muscular movement. Nuclei from such embryos injected into normal early embryos are capable of developing into patches of tissue, some of which produce structures of normal number and size and are presumably diploid, some of which produce increased numbers of smaller-than-normal structures and are presumably haploid, and a few of which are mixed. Haploid tissue with female phenotype found in basistarsus, tergites, and terminalia. Nuclear division cycles of haploid embryos 2.1 min. longer than wild type; such embryos undergo an extra nucleus division in the syncitial blastoderm, possibly to achieve a proper nuclear: cytoplasmic ratio prior to cellularization (Edgar, Kichle, and Schuberger, 1986, Cell 44: 365-72). Haploid cell cultures established from 25 embryos (Debec, 1978, Nature 274: 255-56). Haploid cells and their diploidized derivatives lack centrioles (Debec, Szollosi, and Sollosc, 1982, Biology of the Cell 44: 133-38).