Sensitivity to UV-B radiation is one of the main limitations of biological control of plant pathogens in the field. The effect of UV-B radiation on germination and leaf tissue colonization by the biological control agent Clonostachys rosea was evaluated. There were variations among C. rosea strains in sensitivity to UV-B radiation. The most tolerant strain (LQC62) had relative germination of about 60 % after irradiation of 4.2 kJ m(-2). The deleterious effects of UV-B radiation on C. rosea colonization were overcome by higher conidial concentration. In addition, the tolerance of C. rosea conidia was higher when irradiated over leaf disks compared to agar media, and this is very important information to determine the dose and spray strategies for applying C. rosea in the field.