Cell-free culture supernatants of the entomopathogenic fungus Beauveria bassiana (Bals.) Vuillemin were able to induce mortality in several tick species, including adult Ambylomma americanum (L.), Ambylomma maculatum Koch, and Ixodes scapularis Say. Four lines of experimental evidence support the hypothesis that oxalic acid secretion by B. bassiana, coupled to a reduction in the pH of the medium, act as potent acaricidal factors during pathogenesis. 1) Acaracidal activity of culture supernatants was retained after treatments including boiling and protease digestion, but was lost after dialysis. 2) Metabolite analyses indicated oxalate to be the major secreted organic compound present in the active culture supernatants. 3) Treatment of ticks with the pure compound oxalate at pH 4.0 resulted in almost 80% mortality in adult A. americanum ticks within 14 d, whereas treatment of ticks with oxalate at pH 7.0, or with formate, citrate, or phosphate at both pH 4 and 7 resulted in <10% mortality, even after 28 d. 4) Cell-free culture supernatants from B. bassiana mutants with decreased oxalate production displayed lower acaricide activity than wild type.