Unfed adult Ixodes scapularis Say were treated with spores of the entomopathogenic fungus Metarhizium anisopliae Metschnikoff in the laboratory and in the field. An M. anisopliae suspension containing 4 x 10(9) spores per milliliter caused 96% mortality in the laboratory, versus 53% mortality among field-treated ticks. The LC50 value for unfed adult I. scapularis in the laboratory was 4 x 10(7) spores per milliliter. Our results indicate that M. anisopliae was highly pathogenic to unfed adult ticks and showed potential for controlling questing adult I scapularis.