The dimorphic mycopathogen, Nomuraea rileyi, infects many noctuid defoliators and under proper environmental conditions induces epizootics in insect populations. Nomuraea rileyi isolated from mycosed Spodoptera sp. collected northern Thailand was used to infect lab reared Spodoptera litura larvae. Infected larvae were extracted with dichloromethane and methanol (1:1 v/v) and tested for cytotoxic activity on the Spodoptera frugiperda cell line (Sf9). The most active fraction compound of this extract was isolated and identified to be ergosterol peroxide using a combination of nuclear magnetic resonance techniques, infrared and mass spectroscopy. Various formulations of synthetic ergosterol peroxide were assayed to determine their impacts on the development and survival of test insect and on hemocyte function.