The nar promoter of Escherichia coli was characterized, which is maximally induced under anaerobic conditions in the presence of nitrate. The following results were obtained; Expression of beta-galactosidase was optimal at 1% of nitrate and was not affected much by molybdate; the amount of beta-galactosidase per unit volume was maximal when the nar promoter was induced at OD600 = 1.7, and when anaerobic condition was made by supplying nitrogen gas. At the optimal condition, the ratio of D-galactosidase between before and after induction was approximately 250 and Miller units were approximately 7,500. The results showed that the nar promoter can be used as an inducible promoter.