Powder formulations of three species of entomopathogenic fungi were evaluated for their pathogenic effect upon adult horn flies, Hematobia irritans (L.) (Diptera: Muscidae). Flies were treated with conidia and blastospores of the entomopathogenic fungi Beauveria bassiana (Bals.) Vuill. (strain GHA), Metarhizium anisopliae (Metschnikoff) Sorokin (strain ESCI), and Paecilomyces fumosoroseus (Wize) Brown & Smith (strain ARSEF 3581) in the laboratory. At 4 d postexposure, flies treated with B. bassiana had an average of 98.4% mortality versus 43.5% from treatment with M. anisopliae and 13.0% from treatment with P. fumosoroseus. At 7 d postexposure, flies treated with B. bassiana had an average of 100.0% mortality compared with 73.0% from treatment with M. anisopliae and 33.3% from treatment with P. fumosoroseus. Mean lethal time (LT₅₀) was 2.70, 4.98, and 7.97 d for B. bassiana, M. anisopliae, and P. fumosoroseus, respectively. Entomopathogenic fungi such as B. bassiana and M. anisopliae may have the potential for controlling populations of horn flies. These studies indicate that B. bassiana and M. anisopliae were not only pathogenic to adult horn flies, but they caused mortality in a short time.