The bithorax complex consists of three genes, Ubx, abd-A and Abd-B, which together specify the characteristic development of parasegments 5 to 13 of Drosophila. These genes are structurally homologous; they are of similar size, are transcribed in the same orientation and they all have a homeobox near the 3' end of their transcription unit. Genetic and molecular analyses of Ubx suggest that the gene contains one transcription unit encoding the protein products and at least three cis-regulatory regions. Two of these, abx and bxd, promote the activity of the Ubx transcription unit to the levels appropriate for parasegments 5 and 6, respectively. A third regulatory element, called Cbx-like, prevents the expression of Ubx anterior to parasegment 5. The gene abd-A is not as well known, but genetic and molecular studies indicate at least one cis-regulatory region downstream of the 3' end of the transcription unit. In the gene Abd-B there are two distinct trans-acting elements, called m and r. The m element is a conventional homeotic function, which specifies the identity of parasegments 10 to 13. The r element is specific for parasegment 14 where it suppresses a number of homeotic functions (including m). Molecular analysis indicates that Abd-B contains two transcription units with a common 3' end which correspond to the m and r elements defined genetically.