The homoeotic genes of Drosophila seem to be involved in the establishment of developmental pathways and the specification of different anatomical segment identities. For example, it has been proposed that a function of the Antennapedia+ (Antp+) locus is to direct embryonic cells to follow a mesothoracic developmental pathway. Using cloned cDNA probes of the Antp+ (ref. 6) locus and an improved in situ hybridization method, we found that the neural cells of the embryonic and larval mesothorax possess higher levels of Antp+ transcripts than the neural tissue of the other segments. Here we present further evidence that Antp+ products are associated with formation of the mesothorax, obtained by analysing the distribution of Antp+ transcripts in bithorax mutants. Embryos homozygous for any one of several bithorax mutations display a transformation of the epidermis of the more posterior segments into the homologous tissue of the mesothorax. For example, the metathorax and first seven abdominal segments of embryos that completely lack the bithorax gene complex show a cuticular phenotype characteristic of the normal mesothorax. By in situ hybridization of an Antp+ cDNA probe to tissue sections of these embryos, we have found that the neural cells of the transformed segments accumulate Antp+ transcripts to a level characteristic of the normal mesothorax.