Epidemics of onion leaf spot caused by Botrytis spp. depend on necrotic leaf tissue for inoculum build up in the crop. Ulocladium atrum Preuss. is a strong competitor on necrotic above-ground plant tissues. The potential of the antagonist to reduce colonisation of necrotic leaf tissue by Botrytis spp. and subsequent sporulation was studied in two field experiments. U. atrum colonised necrotic tissues and consistently reduced the sporulation of fungal competitors. Although incidence of Botrytis spp. was low, significantly lower spore loads of Botrytis spp. were found on spore traps located within U. atrum treated onion plots as compared to untreated control plots. The number of leaf spots caused by Botrytis spp. was low in both field experiments but was significantly reduced by 60 fter U. atrum applications in the second experiment. Spraying of the fungicide Ronilan resulted in the same control level. Results show that U. atrum has a potential for biological control against diseases caused by Botrytis spp. in onions.