The anaerobically-regulated nirB promoter was used to express heterologous genes in Escherichia coli. Under anaerobic conditions the promoter was able to express tetanus toxin fragment C at approximately 20% total cell protein (tcp) and the Bordetella pertussis antigen pertactin at greater than 30% tcp. These levels are comparable to those obtained for the same products using the tac promoter. The nirB promoter is very well regulated, giving almost two orders of magnitude increase in fragment C on complete removal of oxygen. The use of this anaerobically-induced promoter in the production of recombinant proteins in E. coli is discussed.