Adaptive response is one of the DNA repair mechanisms. When the cells are pretreated with low doses of alkylating agents, the expression of ada and alkA genes are elevated. These inducible genes make the cells resistant to the exposure of alkylating agents. In order to identify the regulatory sequences for the gene induction, we constructed the pS206 plasmid, which contains alkA-lacZ fused gene. The structural lacZ gene in this plasmid was expressed under the control of alkA promoter. the induced expression of alkA gene was proved to be regulated by the Ada protein, the product of ada gene. The regulatory pattern of the expression was further analyzed by using various sequential deletion mutants of alkA promoter. We concluded that the sequences of regulatory element for this gene induction in alkA promoter might be located between -84 and -49 upstream region from the ATG start codon.