Several modes of action have been proposed to explain the biocontrol capacity of the Fusarium oxysporum protective strain Fo47, especially competition with the pathogen for root colonization. Confocal microscopy has been used to visualize the interactions between the ransformed strains and the tomato root in soil. A disinfested soil was infested by Fo47 GFP, Fol8 DsRed2, or both fungi together at several concentrations before transplanting tomato seedlings. The general pattern of soil and root colonization was similar for the two fungi, but the saprophytic development of Fo47 was faster than that of the pathogenic strain. Conidia, at the surface of soil particles, germinated as early as 18 hours after transplantation of tomato. Hyphae reached the root surface where they created dense networks on the older part of the root, not showing any preferential growth. There was an obvious colonization of soil particles in the rhizosphere. In contrast to what was described in hydroponics, the elongation zone and the apices of tap and secondary roots were never colonized. When the two fungi were introduced together at the same concentration, both fungi were observed together in the older part of the root, but Fo47 was alone in the younger part of the root. When Fo47 was introduced at a higher inoculum concentration (1x105) than Fol8 (1x103), it obviously was dominant. However, it was possible to observe a few zones where both Fol8 and Fo47 were present together at the same spot. Fo47 never totally excluded Fol8 which growth was greatly restricted.