Aceria guerreronis Keifer (Acari: Eriophyidae) is a phytophagous mite that is one of the serious pests of coconut fruits worldwide. Chemical controls have been and continue to be the primary means of control. However, pesticide compatibility with natural enemies is a major concern for IPM systems. The aim of this study was to explore the contact toxicity and egg hatching inhibition of conidia suspension and crude ethyl acetate extract of Hirsutella thompsonii F.E. Fisher against adults and eggs of A. guerreronis. The results indicated that six isolates of H. thompsonii were evaluated for their pathogenicity against eggs and adults of the coconut mite (A. guerreronis), and H. thompsonii TBRC 6805 and TBRC 6933 resulted in the highest mortality (100%) on the 3^rd day post-inoculation (DPI). The crude ethyl acetate extracts 5% (w/v) of H. thompsonii isolates TBRC-BCC 13865 showed the highest mortality to eggs (92%) and adults (98%) of A. guerreronis Keifer, and the LC₅₀ value was 2.629% (eggs) and 1.704% (adults) after 72 h posttreatment. This study shows that H. thompsonii, a fungus well known for controlling biotic stress brought on by insect pests, can also have potential for the development of novel bioacaricides to control A. guerreronis in an integrated pest management program.