Pseudomonas aeruginosa strain P15 produces not only pyocin R1 and phage PS10, but also a substance having a flexuous rod structure, the nature of which is so far unknown. A variant strain (P15-40) was obtained which produced these flexuous particles more effectively than the original strain, and the particles were purified to homogeneity and investigated. Several strains of P. aeruginosa were found to be killed by the particles. It was concluded that the flexuous rod-like particles are not related to pyocin RI or phage PS10, but represent a new pyocin, which we have designated as pyocin Fl. Pyocin Fl showed a different action spectrum and a different pattern on SDS-polyacrylamide gel electrophoresis from either pyocin Rl or phage PS10. The killing activity of pyocin Fl was of single-hit type. The activity was not affected by anti-R1, anti-Rl-core or anti-PS10, or by DNase, RNase, pronase or trypsin, but was completely destroyed by treatment at 70°C for 10min. Some cofactor was required for the adsorption of this pyocin on sensitive bacteria. Another flexuous bacteriocin was also found and named pyocin F2.