Previous attempts to control populations of the black-legged tick (Ixodes scapularis, formerly known as the deer tick, the primary vector of Borrelia burgdorferi, the etiological agent of Lyme disease) have relied on chemical acaricides and generally have been unsuccessful. Several species of entomopathogenic fungi have been utilized successfully to control agricultural pests and, therefore, may be promising alternatives to pesticides in the control of disease vectors like I. scapularis. Laboratory assays have firmly established the virulence of several entomopathogenic fungi to I. scapularis, but lower humidity in the field may fail to trigger spore germination. Pathogenicity may therefore be reduced in situ. We conducted studies comparing the efficacy of Beauveria bassiana as a control agent of I. scapularis in the laboratory to its efficacy under natural field conditions. The commercially available suspension of B. bassiana spores, Botanigard ES, was pathogenic to 86% of I. scapularis adults in the laboratory. However, when we applied this suspension to field plots, the mortality rate of adult ticks was not significantly different from that in control plots, based on drag sampling (p=0.238) and tick survival cages (p=0.345). Similarly, preliminary results suggest no difference between treatment and control plots in mortality rates of nymphal and larval ticks (p=0.108, 0.186, respectively). We are now using diagnostic PCR to assess whether ticks from treated plots have a higher rate of contact with B. bassiana than do ticks from control plots. Such a result would indicate that environmental conditions, not ineffective application techniques, are responsible for the low field mortalities that we observed.