Null mutations in the Drosophila melanogaster homeotic gene proboscipedia (pb) cause transformation of the adult labial palps to legs. The similar phenotype produced by mutations in the Tribolium castaneum homeotic complex (HOMC) gene maxillopedia (mxp) has led to suggestions that the two genes may be orthologous. We have cloned the Tribolium ortholog of pb, which predicts a protein with a homeodomain identical to that of Drosophila Pb. The two proteins also share several additional regions of identity, including an N-box, a motif unique to Pb orthologs. We have identified a frameshift mutation within Tribolium pb associated with an mxp null mutation, demonstrating that Tribolium pb corresponds to the mxp genetic locus. Thus, we will refer to the cloned gene as mxp. In addition, we have begun to construct a molecular map of the Tribolium HOMC. Two overlapping BAC clones which span the mxp locus also include the Tribolium labial ortholog (Tclabial) and part of Tczerknüllt, indicating that the order of these genes in the HOMC is conserved between Drosophila and Tribolium.