The amino acid sequence deduced from the L1 beta-lactamase gene of Xanthomonas maltophilia shows a significant variation from that of the CphA and Blm metallo-beta-lactamases of Aeromonas hydrophila and Bacillus cereus, respectively. Whilst the N-terminus of the L1 protein shows some similarity, particularly at the histidine residues previously suggested as a zinc-binding motif, the C-terminus of the protein demonstrates very little similarity. Such differences amongst this group of enzymes would argue for at least three subclasses within the Group 3 beta-lactamases. However, in order to predict their phylogenetic ancestry more sequence data are required from other possible metallo-beta-lactamases.