The antibiotic, 2,4-diacetylphloroglucinol (Phl), is produced by a range of naturally isolated fluorescent pseudomonads, found in disease suppressive soils. The natural isolate, P. fluorescens F113, protects pea plants from the pathogenic fungus, Pythium ultimum, by reducing the number of pathogenic lesions on the plant's roots. This beneficial effect was however, outweighed by the F113 causing an overall reduction in the emergence of the pea plants in the infected soil. The gene locus responsible for the Phl production was shown to be functionally conserved between the P. fluorescens F113 and another Phl producing organism, P. fluorescens Q2-87. Following identification of this functional sequence homology, the genes were isolated from F113, by optimised, long PCR. The 6.7-kb gene cluster was inserted into the chromosome of a non-pathogenic P. fluorescens, SBW25, which can effect biological control against the plant pathogen, Pythium ultimum through competitive exclusion of the fungus, by means of its strong colonising competence. The insertion was a targeted, homologous recombination designed to insert the Phl coding genes, from the F113, into a non-essential, lacZY coding region of the SBW25 chromosome. The transformed strains of SBW25 assumed two different morphological appearances. The morphological changes were noted at a ratio of 1:1 of normal morphology and altered morphology. Transformation of SBW25 with the Phl locus without this repressor element led to transformants with only normal morphology. All transformants were able to suppress P. ultimum through antibiotic production following the Phl transformation. However, the fitness of the transformants was reduced in flask culture, at 30°C, against the un-transformed SBW25. The organisms transformed with the entire Phl locus were seen to clump together in the culture media. The strain transformed with the Phl locus lacking the repressor element behaved normally. When inoculated on pea seedlings, the strain containing no repressor element behaved similarly to the F113, causing lower pea seed emergence. A transformant containing the entire Phl genetic locus had not lost its environmental competence on the pea roots, maintaining a high population, but was unable to maintain a high population in the surrounding soil.