Abstract: Human activities are altering the concentrations of ozone in the troposphere and hencein the incidence of ultraviolet-B (UV-B) on Earth’s surface. Although representing only fivepercent of UV-B radiation striking the Earth’s surface, this radiation has the potential to causeeffects on biologically active molecules. Sensitivity to UV-B radiation is one of the limitations ofbiological control of plant pathogens in the field. The objectives of this work were to evaluate theeffects of UV-B on several isolates of Clonostachys rosea, and the ability of an isolate ofC. rosea, previously selected for its tolerant to UV-B radiation, to control Botrytis cinerea onstrawberry leaves in controlled conditions (strawberry leaf discs). The germination of C. roseaconidia was inversely proportional to the irradiance. The most tolerant strain (LQC62) hadrelative germination of about 60% after irradiation of 4.2kJ/m2, and this strain was selected to beused in the subsequent studies. The data showed that even with exposure to UV-B radiation,C. rosea LQC62 controlled the pathogen. Conidial concentrations of strain LQC62 above 105conidia/ml showed higher tolerance to UV-B radiation and increased ability to control more than75% of the B. cinerea even with exposure to radiation. According to our results, in addition toshowing less growth under UV-B, conidia of C. rosea had lower antagonistic ability. Furtherstudies are needed to observe the tolerance of B. cinerea conidia to UV-B radiation and therebyprove that an environment with increased UV-B radiation may be favoring the pathogen due to alower ability of C. rosea to control the pathogen in conditions of increased UV-B.