The twist zygotic gene appears to be involved in the establishment of the dorso-ventral pattern in Drosophila embryos. Homozygous twist embryos are partially dorsalized, their gastrulation is abnormal, and they fail to differentiate mesoderm. We determined the temperature-sensitive period of twist around the gastrulation time, and we isolated the gene. A 300 kb chromosomic walk allowed the detection of the 70 kb deletion that delimits the twist region in Df(2R)twiS60. Southern blot analyses of 21 EMS induced twist allele DNAs and systematic Northern blot analyses all over this 70 kb region lead to the localization of the twist gene: within about 10 kb at the left border of the deletion, 2 twist alleles show each a small deletion that uncover a transcription unit whose expression occurs about at the time of gastrulation.