To identify genes involved in the decolorization of crystal violet, we isolated random mutants generated by transponson insertion in crystal violet-declorizing bacterium, Citrobacter sp. The resulting mutant bank yielded mutants with six distinct phenotypes, and Southern hybridization with a Tn5 fragment as a probe showed a single hybridized with six distinct phenotypes, and Southern hybridization with a Tn5 fragment as a probe showed a single hybridized band in the mutants Ctg 2, 5 an 6, whereas two and three bands were detected in Ctg1, 4 and 3, respectively. Tn5-inserted genes were isolated and the DNA sequence flanking Tn5 was determined. From comparison with a sequence database, putative protein product encoded by ctg 5 was identified as E. coli maltose transproter(Mal G) homolog, whereas the deduced amino acid sequence of the other ctg genes did not show any significant similarity with any DNA or protein sequency. Therefore, these results indicate that the other ctg genes except ctg 5 encode new proteins responsible for decolorization of crystal violet.