Investigation of the expression of the riboflavin (rib) genes, which are found immediately downstream of luxG in the lux operon in Photobacterium phosphoreum, provides more information relevant to the evolution of bioluminescence, as well as to the regulation of supply of flavin substrate for bacterial bioluminescence reactions. In order to answer the question of whether or not the transcriptions of lux and rib genes are integrated, a transcriptional termination assay was performed with P. phoxphoreum DNA, containing the possible stem-loop structures, located in the intergenic region of luxF and luxE ($Omega$$\_$A/), of luxG and ribE ($Omega$$\_$B/), and downstream of ribA ($Omega$$\_$c/). The expression of the CAT (Chloram-phenicol Acetyl Transferase) reporter gene was remarkably decreased upon the insertion of the stem-loop structure ($Omega$$\_$c/) into the strong lux promoter and the reporter gene. However, the insertion of the structure ($Omega$$\_$B/) into the intergenic region of the lux and the rib genes caused no significant change in expression from the CAT gene. In addition, the single stranded DNA in the same region was protected by the P. phosphoreum mRNA from the Sl nuclease protection assay. These results suggest that lux genes and rib genes are part of the same operon in P. phosphoreum.