Bed bugs Cimex lectularius L. were exposed to conidia (spores) of the entomopathogenic fungus Metarhizium anisopliae by feeding, aerosol spray, or contact with a treated surface. Feeding experiments demonstrated that bed bugs were innately susceptible to this fungus. However, only at 98% humidity were mortality rates high, regardless of whether bed bugs were sprayed with a fungal solution or contacted a treated surface. Mortality in treated bed bugs at ambient humidity did not increase when these bed bugs were kept in aggregation with other bed bugs that had recently blood fed to repletion. Based on these laboratory studies, we conclude that M. anisopliae is a poor pathogen for use in control of bed bugs, particularly at humidities that would likely be encountered under field conditions.