Pathogenicity of a native strain of Beauveria bassiana Vuill. (Ascomycota: Hypocreales) was evaluated on third-and fifth-instar larvae of tobacco budworm, Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae). Larvae were immersed in 1×10⁸ conidia ml^-1. After 72 hours, the percentage of each treated larval instar that died was 71 and 55%, respectively, while mortality of the nontreated check was 8 and 11%. The fungus was found across the cuticle and grew in the hemocoel 48 to 60 hours after inoculation of third-instar larvae and 60 to 72 hours for fifth instars. Fungal infection events also were studied in tobacco budworm larvae. The microscope and staining procedure showed mycelia grew as filamentous hyphae on the surface and penetrated the cuticle and spiracles; this phenomenon killed larvae. Inside the larvae, fungus grew as filamentous hyphae, but blastospores were not noticed. Results showed many young larval stages died and specific infection events in tobacco budworm compared to other data reported. Results suggested this native strain has potential as a bioinsecticide for tobacco budworm.