Camponotus ants harbor the obligate intracellular endosymbiont Blochmannia in their midgut bacteriocytes, but little is known about intestinal bacteria living in the gut lumen. In this paper we reported the results of a survey of the intestinal microflora of Camponotus japonicus Mayr based on small-subunit rRNA genes (16S rRNAs) polymerase chain reaction (PCR)-restriction fragment-length polymorphism analysis of worker guts. From 107 clones, 11 different restriction fragment-length polymorphism profiles were identified, and sequences blasting analysis found these represent four types of bacteria. Most (91.6%) of the clones were "Candidatus Blochmannia", the obligate endosymbionts of Camponotus ants, and 6.5% of the clones were "Candidatus Serratia symbiotica", a secondary endosymbiont of aphids; the remaining 2% clones were Fructobacillus fructosus and uncultured Burkholderiales bacterium, respectively. These results show that the diversity of gut bacteria in C. japonicus was low. "Candidatus Serratia symbiotica" was identified from Camponotus ants for the first time, an interesting result because Blochmannia's closest bacterial relative is also in the genus Serratia. This discovery supports the scenario that consumption of aphid honeydew or tissue provides an initial step in the evolution of an advanced symbiosis, and suggests that Camponotus ant could acquire other secondary endosymbionts from Hemiptera host through their diet. In addition, Burkholderiales bacterium also was identified from the gut of C. japonicus for the first time, and whether it is a nitrogen-recycling endosymbiont in Camponotus ants needs to be investigated further.