Erwinia carotovora subspecies atroseptica is the agent of soft rot of potato and causes important crop damage in Europe. Synthetic 2,4-diacetylphloroglucinol (DAPG) inhibited the growth of E. carotovora subsp. atroseptica under in vitro conditions and Pseudomonas fluorescens F113, which produces DAPG, was studied for biocontrol of a carotovora subsp. atroseptica. Wild-type F113 (or the spontaneous rifampicin-resistant mutant F113Rif) inhibited growth of E., carotovora subsp. atroseptica on solid medium, displayed bactericidal activity towards the pathogen in liquid medium, and prevented Erwinia-mediated rotting of wounded potato tuber under in vitro conditions. F113Rif reduced the population size of E. carotovora subsp. atroseptica in soil and on potato tuber dice in competition experiments carried out with unplanted soil and soil planted with diced potato tubers, respectively. Go-inoculation of potato tuber seeds with F113Rif and E. carotovora subsp. atroseptica reduced Erwinia contamination of the seed tubers compared with single inoculation with the pathogen. F113G22 is a Tn5::lacZY-induced DAPG-negative biosynthetic derivative of F113 and showed no antibiosis towards E. carotovora subsp. atroseptica in vitro. In contrast to F113Rif, F113G22 did not inhibit Erwinia-mediated rotting of wounded potato tuber in vitro, did not influence survival off. carotovora subsp. atroseptica in unplanted soil or soil planted with potato tuber dice and did not reduce Erwinia contamination of potato seed tubers. F113G22(pCU203) is a complemented derivative with restored DAPG-producing ability. F113G22(pCU203) had similar effects against E. carotovora subsp. atroseptica as F113 (or F113Rif) under in vitro conditions and in soil microcosms. The results indicate that P. fluorescens F113 is a promising biocontrol agent against the potato soft rot agent E. carotovora subsp. atroseptica and suggest that the pseudomonad's ability to produce DAPG is a key factor in its inhibition of the pathogen.