The development of BCAs represents an attractive alternative to fungicides for the protection of crops against plant pathogens but the durability of this method has not been studied in details. The objective of the present work was to estimate the risk of loss of biocontrol efficacy towards Botrytis cinerea, by evaluating the sensibility of various isolates of the pathogen to the biocontrol agent Microdochium dimerum. The protective efficacy of M. dimerum was evaluated on tomato plants against 41 strains of B. cinerea differing in their geographic origin and host of isolation. To this end, whole tomato pruning wounds and detached stem sections were concomitantly inoculated with B. cinerea and with M. dimerum. Lesion expansion was recorded daily from the 3rd to the 7th day after inoculation. Due to the very high level of efficacy against all tested strains of B. cinerea when M. dimerum was used at the recommended dose, it was necessary to reduce the dose of application 10-fold to assess the diversity of sensitivity of B. cinerea to this BCA. In these conditions, a wide range of sensitivities were observed among strains of the pathogen tested with protection levels ranging from 0 to 100% (mean = 53 ± 4%; median = 49%). A correlation was observed between the level of aggressiveness of a strain to tomato and its sensitivity to the biocontrol agent (assessed by the protection level). It reveals the importance of considering several strains of the pathogen when screening for biocontrol agents, to obtain a good representation of the pathogen population and thus take into account the potential durability of biocontrol.