The iron response in a number of bacterial systems is mediated by fur (ferric uptake regulation)-like regulatory systems. We have cloned and characterized a gene from Neisseria meningitidis that was homologous to Escherichia coli fur. This clone was capable of modulating expression from both E. coli and neisserial iron-regulated promoters in response to iron, and it produced a protein that reacted with anti-E. coli fur serum. Although the DNA and predicted amino acid sequences were very similar to those of four other published fur homologues, meningococcal fur was the most divergent of the group. Inability to construct a meningococcal fur mutant suggested that fur may be essential in this species.