The bithorax genes are a group of homoeotic genes of Drosophila whose function is related to the control of segment development. They form a gene complex in which at least seven different loci have been identified. Mutations at each of these loci produce a specific homoeotic transformation whereby a segment is transformed either in part or completely into another. In embryos completely deficient for the bithorax genes, all the thoracic and abdominal segments resemble mesothoracic segments. This observation, together with the segmental specificity of the mutant phenotypes, led Lewis to suggest a model of genetic control in which the type of development in each segment is specified by the activation of a fraction of the total number of bithorax genes. Thus, lack of activity of the bithorax genes results in mesothoracic development, activation of the bithorax (bx+) and postbithorax (pbx+) genes produces metathorax (anterior and posterior respectively), and activation of these two genes with bithoraxoid (bxd+) produces the first abdominal segment and so on. Although this model explains the genetic and developmental data, it leaves unexplained formation of the prothoracic segment. We now describe two unexpected results: deficiencies for bithorax genes can lead to segments being transformed into prothorax, and there is a temporal sequence in the function of the bithorax system.