The effects of two uv irradiances (920 and 1200 mW m−2 weighted irradiance) on the conidia and germlings of the ARSEF 2575 and ARSEF 23 strains of M. anisopliae were studied. Conidia were exposed to the two irradiance levels for 1, 2, 3, 4, 6, or 8 h. The 30% increase (from 920 to 1200 mW m−2) in uv irradiance caused a significant decrease in culturability following all periods of exposure and reduced the 50% lethal time (TL50) 36% for strain 2575 and 48% for strain 23. Exposure to uv radiation of only 1 h caused a delay of several hours in the germination of surviving conidia. Longer periods of exposure delayed germination for days, demonstrating that, depending on the dose, the fungus may require a long period of time to recover and to resume germination. The results demonstrated the inability of the fungus to germinate during direct exposure to the uv-B portion of simulated sunlight. Both strains showed a transitory increase in uv tolerance during germination. The beginning of germination increased uv tolerance. However, starting on the 6th hour of germination, a decrease in tolerance was observed, indicating that uv tolerance varies as a function of physiological state and cell-cycle phase.