Nymphal and adult ticks from three different tick species, Dermacentor variabilis Say, Ixodes scapularis Say, and Rhipicephalus sanguineus Latrielle, were treated with conidia and blastospores of the entomopathogenic fungi Beauveria bassiana (Bals.) Vuill. and Metarhizium anisopliae Metschnikoff. Dose-response experiments indicated that a critical concentration of fungal spores is required for infection and mortality. Over a 28-d time course, fungal suspensions of either B. bassiana or M. anisopliae at 10(8) conidia/ml resulted in 50-70% mortality in adult I. scapularis and R. sanguineus, but <20% mortality in D. variabilis ticks. R. sanguineus nymphs were highly susceptible to both entomopathogenic fungi, displaying >60% mortality within 14 d postinfection and >90% mortality within 21-28 d postinfection. D. variabilis nymphs also were more susceptible than their corresponding adults, displaying mortalities ranging from 20 to 40% 28 d postinfection. I. scapularis nymphs, however, seemed to be slightly less susceptible than adults (45% mortality, 28 d postinfection). The addition of nutrients to fungal cell suspensions did not have any noticeable effects on mortality toward any of the tick species tested. Significant mortality against D. variabilis adults (approximately 65%) was noted only when B. bassiana fungal cells with growth media carryover were used as the inoculum against the ticks. Entomopathogenic fungi such as B. bassiana and M. anisopliae may have the potential for controlling populations of I. scapularis and R. sanguineus, and under certain conditions D. variabilis. Our results indicate that inoculum conditions can greatly affect successful virulence and subsequent mortality.