The osteolytic toxin of Pasteurella multocida induces bone resorption in vivo and in vitro (Foged et al., 1988; Kimman et al., 1987). In this report the toxin-encoding toxA gene is sequenced, and the deduced primary structure of the toxin shows a protein of 1285 amino acids, containing a striking homology to a metal-binding motif. Evidence that expression of the toxA gene is repressed at a transcriptional level in Escherichia coli is presented. Repression could be abolished either by deletion of a region upstream of toxA, or by a putative frame-shift mutation in the same region. The repressor protein encoded within this region was efficient in trans, and was named TxaR.