The colonization of barley roots by Fusarium culmorum was studied in the sterile vermiculite using the method of fluorescent antibodies. The influence of antagonistic bacterium Pseudomonas fluorescens on the root colonization by fungus was established. The fungus density on roots was increased gradually when vermiculite was inoculated by F. culmorum only. When vermiculite was inoculated by F. culmorum with P. fluorescens the fungus density on roots was changed abruptly. It has been established that the sites of primary root colonization by fungus are in the zone of root hairs mainly. When fungus and bacterium were presented on roots, for two first days F. culmorum has colonized not only root hairs, but also the zone of elongation. Introduction of P. fluorescens into vermiculite has led to decrease of barley root rot intensity.