The virulence of Hirsutella thompsonii (Fischer) to Brevipalpus phoenicis (Geijskes) was evaluated in laboratory, grown on complete and solid culture media (MC-S); complete and liquid culture media (MC-L); rice (APC) and powdered rice (APC-SM). Adults were confined to arenas prepared with citrus leaves in acrylic dishes containing water-agar. Conidial suspensions were prepared at different concentration (3.2 x 10(5) to 1 x 10(7) spores/ml) and applied on mites to establish the table curve-response on fourth day. For field evaluation, adults were maintained in arenas prepared with fruits which were placed in plants. In this test, four treatments were tried: H. thompsonii cultured on rice (APC) at two concentrations (20 kg/ha and 10 kg/ha), H. thompsonii produced by liquid fermentation (MC-L) (5 L/ha) and control (sterile water). Adult survival, number of eggs and nymphs per fruit were observed 10 and 20 days after the fungus application. The lowest LC25 value calculated was from pathogen produced in MC-S (1.9 x 10(5) conidia/ml).The LC25 values calculated to APC and APC-SM did not differ statistically. The LC25 values to MC-L and MC-S were 1.9 x 10(6) infective cells/ml and 2.2 x 10(5) conidia/ml. In the field, concentration and time to death differed between treatments and control. The applications resulted in reduction of adult survival and number of eggs.