Ralstonia solanacearum is an important phytopathogen that attacks many crops and causes bacterial wilt. The ability to produce extracellular polysaccharides (EPS), various kinds of cell wall degrading enzyme (CWDEs) and some virulence factors is important for R. solanacearum to cause disease, which are positively regulated by a transcriptional regulator PhcA, which is also known to activate many other virulence genes in R. solanacearum. PhcA is activated by two component regulatory system PhcS/PhcR via quorum sensing autoinducer 3-hydoxy palmitic acid methyl ester (3-OH PAME). Therefore, cell surface conditions of the bacteria are closely related to the virulence. Recently, various phages that infect R. solanacearum strains were isolated and characterized. Two groups of these phages, ?RSM-type and ?RSS-type phages, are filamentous phage with a circular single-stranded (ss) DNA genome. These phages showed contradictory effects on host virulence, that is loss of virulence and virulence enhancement. In this work, I further investigated how these filamentous phages effect on host-virulence after infection. I also studied their potential to be utilized as biological control agents.