Pseudomonas fluorescens C7R12 was shown to promote arbuscular mycorrhization and to act as a ‘Mycorrhiza Helper Bacteria’ (MHB). The complex interactions between this model strain, Arbuscular Mycorrhizal (AM) fungi and host-plants were analysed by assessing the specificity of these interactions and by characterizing bacterial cell organization on mycorrhizal roots. Evaluation of the interaction specificity relied on the comparison of the bacterial effect on (i) the in vitro saprophytic growth of Glomus mosseae and Gigasopora rosea and (ii) the root colonization of two different plant species (Medicago truncatula and Lycopersicon esculentum) by the two AM fungal species. Characterization of bacterial cell organisation relied on microscopic observations (immunofluorescence technique and confocal laser scanning microscopy) made on mycorrhizal (myc+, G. mosseae) and non-mycorrhizal (myc-) roots. The results obtained showed that P. fluorescens C7R12 promoted the in vitro saprophytic growth of G. mosseae but not that of Gi. rosea. This bacterial strain also promoted mycorrhization of medic and tomato with G. mosseae, but not that with Gi. rosea for any of the plants tested. Microscopic observations allowed the identification of five types of cell organization (Organization Types OT): small single cells, large single cells, cells by pairs, cells in micro-colonies, and cells in strings. The frequencies of each OT differed significantly on myc+ and mycroots. Bacterial cells were more frequently single on myc+ roots, and in micro-colonies and strings on myc- roots, suggesting that they were dividing more frequently on myc- than on myc+ roots. Indeed, the root area covered by bacterial cells was significantly higher on myc- than on myc+ roots. Taken together, these results indicate that promotion of AM was fungal but not plant specific and suggest that C7R12 cells were more stressed on myc+ than on myc- roots.