The sigma subunit of Xanthomonas oryzae pv. oryzae is disassociated from host RNA polymerase after phage Xp10 infection. To clarify the possible mechanism for this observation, sigma subunit was purified and an antiserum against sigma subunit was prepared. Immunoprecipitation of RNA polymerase by the anti-core RNA polymerase antiserum, followed by immunoblotting with anti-sigma subunit antibody, revealed that sigma subunit was lost from RNA polymerase within 10 minutes after Xp10 infection. Loss of sigma subunit was not observed under other stress conditions including heat and cold stress, starvation and growth to stationary phase. Two-dimensional immunoblotting analysis did not reveal any covalent modification of either sigma subunit or RNA polymerase after Xp10 infection. These results suggest that separation of th subunit from RNA polymerase may be due to competition with other binding factors.