Rhizobacteria (2808) were isolated on Pseudomonas-selective S1 medium from two maize inbred lines and from their hybrid at three plant growth stages. Positive phlD hybridization was found for 364 of them. The PhlD+ isolates were significantly more numerous in the rhizosphere of the hyrid than in those of parental lines. Furthermore, the frequency of PhlD+ was significantly higher for the hybrid at the flowering stage. An amplified rDNA restriction analysis showed that the hybrid genotype also increases the genetic diversity of PhlD+ populations when compared with its inbred parent lines, and this could be an effect of heterosis. Influence of the hybrid on the frequency and diversity of the bacterial PhlD+ population varied along the plant growth stage.