Abstract: For studying the bioefficacy of six isolates of Hirsutella thompsonii Fisher a fungal pathogen of eriophyid mites collected from different agro-climatic regions of India, fungal suspension was sprayed on fruits and leaves infested with healthy citrus rust mite, Phyllocoptruta oleivora Ashmead and two spotted spider mite, Tetranychus urticae Koch. The results revealed that all isolates of H. thompsonii were effective against citrus rust mite. HtEMC isolate of H. thompsonii brought about maximum mortality of 48.05% ten days after treatment followed by HtCRMB (46.87%), HtCRMK (44.27%), HtCRMC (42.09%), HtPMG (39.82%) and HtPDBC (31.22%), however, this difference was not significant statistically. These isolates were also found to be effective against adult two spotted spider mite. HtEMC isolate of H. thompsonii brought about maximum mortality of 83.33% after eight days of treatment, followed by HtCRMB (70.85), HtCRMK (39.48), HtPMG (33.05), HtCRMC (31.86) and HtPDBC (19.64). The HtCRMB isolate of H. thompsonii was found to be effective against nymphal stage of the two spotted spider mite with 88.85% mortality six days after treatment. These results indicate that H. thompsonii isolates exhibit differences in their virulences, suggesting the requirement for screening the H. thompsonii isolates for their effectiveness against mite species before recommending as biological control agent.