Red flour beetle, Tribolium castaneum (Herbst), is a major pest of stored and processed grains that is tolerant of Beauveria bassiana (Balsamo) Vuillemin under most conditions. Laboratory assays demonstrated that desiccating conditions improved the efficacy of the fungus. For T. castaneum larvae exposed to B. bassiana at different vapor pressure deficits (VPDs), the LC₅₀ potency ratios were 2.3 for 2.42 kPa and 3.9 for 3.31 kPa compared with 1.06 kPa. There were significant effects of moisture on B. bassiana-associated mortality even with brief exposure to low VPD before or at the beginning of exposure to fungus. When T. castaneum larvae were held at 3.31 kPa 1 d before exposure to B. bassiana and/or in the first 1 or 2 d of exposure to the fungus, there was significantly greater mortality for all of the low moisture treatments than occurred with continuous incubation at 1.06 kPa. B. bassiana efficacy for adult T. castaneum was significantly better for VPDs of 3.56 or 3.78 than 1.06 kPa. Weight gain over 4 d of incubation with moisture held constant was significantly less at a VDP of 3.31 kPa than at 1.87 or 1.06 kPa. Treatment with fungus did not affect weight gain significantly. There was no significant difference in larval water content among treatments.