Objective: estimate the risk of loss of biocontrol efficacy towards Botrytis cinerea, by evaluating the sensibility of 41 isolates of the pathogen to the biocontrol agent Microdochium dimerum on tomato plants. Very high level of efficacy against all tested strains of B. cinerea when M. dimerum was used at the recommended dose. At 10-fold reduced dose of M. dimerum, wide range of sensitivities observed among strains of the pathogen tested with protection levels ranging from 0 to 100% (mean = 53 ± 4%). Negative correlation between the level of aggressiveness of a strain to tomato and its sensitivity to the biocontrol agent. Importance of considering several strains of the pathogen when screening for biocontrol agents, to obtain a good representation of the pathogen's population and thus take into account the potential durability of biocontrol.