We report the isolation and characterization of the Hirudo medicinalis homoeobox gene Lox2. Sequence analysis shows that it contains a region that has homology to Drosophila and vertebrate homoeodomains of the Antennapedia class. In addition, Lox2 shares homology with sequences in the bithorax complex Ultra-bithorax (Ubx) and abdominal A (abdA) genes in a region adjacent to the C-terminus of the homoeodomain. Whole mount in situ hybridization of embryos of various ages demonstrates that during early development this gene has temporally and spatially restricted patterns of expression that resemble those of the homoeotic genes of the Drosophila bithorax complex and of many vertebrate homoeobox genes. The largest accumulation of transcripts was seen in the posterior two-thirds of the developing leech central nervous system in 7-14-day-old embryos. Adult leeches also express Lox2. We propose that in Hirudo, Lox2 represents the ancestral gene of the Ubx and abdA genes of the bithorax complex of Drosophila.