A Pseudomonas 2,4-diacetylphloroglucinol (DAPG)-producing population that occurred naturally on the roots, in rhizosphere soil of Zea mays and in the nonrhizosphere soil was investigated in order to assess the microbial diversity at five stages of plant growth. A total of 1,716 isolates were obtained, and 188 of these isolates were able to produce DAPG. DAPG producers were isolated at each stage of plant growth, indicating that the maize rhizosphere is colonized by natural DAPG producers throughout development. The frequency of DAPG producers was very low in the first stage of plant growth and increased over time. An analysis of the level of biodiversity of the DAPG producers at the species level was performed by comparing the AluI restriction patterns of the 16S ribosomal DNAs (rDNAs) amplified by PCR from 167 isolates. This comparison allowed us to cluster the isolates into four amplified rDNA restriction analysis (ARDRA) groups, and the main group (ARDRA group 1) contained 89.8% of the isolates. The diversity of the 150 isolates belonging to ARDRA group 1 was analyzed by the random amplified polymorphic DNA (RAPD) technique. An analysis of RAPD patterns by a molecular variance method revealed that there was a high level of genetic diversity in this population and that the genetic diversity was related to plant age. Finally, we found that some of the DAPG producers, which originated from all stages of plant growth, had the same genotype. These DAPG producers could be exploited in future screening programs for biocontrol agents.Some rhizobacteria, which are commonly called plant growth-promoting rhizobacteria, interact with plant roots and protect the roots against pathogenic microorganisms (26). Pseudomonads are common members of the plant growthpromoting rhizobacterial microflora in the rhizospheres of protected plants (13,25,42). The ability of pseudomonads to suppress soilborne fungal pathogens depends on their ability to produce antibiotic metabolites, such as pyoluteorin, pyrrolnitrin, phenazine-1-carboxylic acid, and 2,4-diacetylphloroglucinol (DAPG) (14,32,38,47,49,55,57). One of these metabolites, DAPG, is a major factor in the biological control of a range of plant pathogens (7,23,24,57). The antibiotic DAPG is produced by pseudomonads of worldwide origin, and its biosynthetic locus is conserved in pseudomonads obtained from diverse geographic locations (25,40). Bacteria that produce DAPG play a key role in agricultural environments, and their potential for use in sustainable agriculture is promising. However, introduction of these bacteria in the field often fails because the organisms are not able to recolonize the roots or colonize the roots but do not produce antibiotic compounds in the new environment (11,41).It is well known that root exudates are sources of nutrition for rhizosphere microorganisms (6, 43). The composition of root exudates is affected by the stage of plant development (18), which results in changes in the patterns and activities of rhizobacterial populations (9, 18, 33). Thus, t...