Abstract: Verticillium longisporum is an amphidiploid fungal pathogen found in three different lineages putatively derived from three independent hybridization events. The lineage A1/D2 has only been found in horseradish in the USA, it mostly exhibits low level aggressiveness in its host and is apathogenic in most Brassica crops. Apathogenic strains of plant pathogens have proven in the past to be candidates for biological control of aggressive strains of that particular fungal species. Thus, the biocontrol potential of this V. longisporum lineage was tested with root dip-inoculation of oilseed rape (B. napus) seedlings, where A1/D2 isolates were separately inoculated a week prior or simultaneously to the inoculation with an aggressive isolate of V. longisporum. A reduction of symptoms was observed for both types of inoculation with A1/D2. Observations with confocal microscopy of the A1/D2 interaction with roots of oilseed rape seedlings suggest that its biocontrol effect might be due to a competition for space at the root surface level, indicated by ample sporulation and mycelial growth on the root.