none Laboratory experiments were conducted to determine the histopathological effects of pathogenic nonoccluded baculovirus (Mc-NOBV) in Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), a potential parasitoid for the control of heliothine species of Lepidoptera. This virus is rod-shaped and has a single envelope. Phosphotungstic acid negatively stained virions measure approximately 75 times 270 and 65 times 260 nm in Spurr's resin thin section. The electron-dense nucleocapsids are approximately 55 times 235 nm in 2% phosphotungstic acid preparation and 50 times 225 nm in principal sites of viral infection. Some viral particles thin section. Fat body and midgut epithelium are the tubules, tracheal matrix, hemocoel, epidermis, muscle, eggs, and reproductive systems of both sexes of M. croceipes. The Mc-NOBV is synthesized in the virogenic stroma (VS). Virions in VS of fat body and midgut epithelium are arranged in a paracrystalline-like structure and are often found dispersed throughout the entire nuclear matrix. Virions in other infected tissues are arranged in irregular patterns. Infected digestive system and fat body cells show noticeable histopathological changes, while other tissues and organs exhibit only slight or no apparent abnormalities. Histopathology of the hypertrophic nuclei, mainly in fat bodies and midgut epithelium, is the first sign of infection in Mc-NOBV. Disintegrated, vacuolated, and ruptured cells and their organelles are first observed 4 days after inoculation.