Previously undiscovered isolates of Beauveria bassiana (Balsamo) Vuillemin (Ascomycetes: Clavicipitaceae) able to control Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae) were obtained for the first time in Argentina. The isolates were selected from three sources: 1) soil samples from the provinces of Corrientes, Formosa, and Chaco, where ticks are endemic; 2) dead female ticks; and 3) the fungal collection from the Entomopathogenic Fungi Laboratory of IMYZA-INTA Castelar. To select the isolates, population parameters were estimated, LC₅₀ values of the most virulent isolates were calculated, and fungi-acaricides compatibility assays carried out. Isolates B. bassiana 259 and 98 were the most virulent and effective to reduce the number of eggs, the percentage of larval hatching, and parameters r_m (natural intrinsic growth rate) and λ (infinite growth rate) of Rh. (Bo.) microplus populations. The values of LC₅₀were 1 × 10⁷ and 1.15 × 10⁷, respectively, when applied to Rh. (Bo.) microplus eggs. In addition, they were compatible with acaricides. A novel methodology to evaluate the entomopathogenic activity of fungi on Rh. (Bo.) microplus ticks is introduced.