While gene products and metabolites of insect symbiotic bacteria may act as useful resources for insect-microbe studies and medicinal use, it is usually difficult to obtain the insect symbionts to some extent in quantity because most of them are unculturable. In this study, the possibility of using bacterial artificial chromosome (BAC) libraries as a heterologous gene expression tool for the discovery of novel symbiont metabolites was evaluated. A BAC library was constructed from the symbiont purified from the posterior midgut cecum of the stink bug Plautia stali . The BAC library, which consisted of 513 clones with an average insert size of 41 kb, represented greater than five-fold coverage of the genome. The ability of the BAC clones to express plural genes from large-sized insert DNA in Escherichia coli was examined by the growth of BAC-transformed leu operon-deficient DH10B cells on M9 minimal medium supplemented with glucose. Two BAC clones complemented leucine deficiency in DH10B cells; the clones contained the leu operon of the symbiont chromosome. The P. stali symbiont genes introduced into the BAC vector are functional in E. coli , and these genes are expressed in an operon unit. BAC libraries can be used to generate gene product- and metabolite-libraries, facilitating to characterize potential metabolites of the P. stali symbiont.