A nonoccluded filamentous virus (CmFV) was found in the reproductive tract of females in two of five laboratory colonies of Cotesia marginiventris established from parasitized lepidopteran larvae collected in the southeastern United States. Nucleocapsids of CmFV were 35-39 nm in diameter and had electron-dense cores 21-24 nm in diameter. CmFV virions with a single envelope measured 60-70 nm in diameter; double enveloped virions were approximately 90 nm in diameter. The unit length was not determined. CmFV replicated in epithelial cells of the common and lateral oviducts and rarely in small nests of cells in the calyx. CmFV was also transmitted to larvae of the fall armyworm, Spodoptera frugiperda, during parasitization. The ability of C. marginiventris to transmit CmFV increased from Day 1 to Days 3-4 postemergence. A polydnavirus (PV) containing multiple nucleocapsids per envelope occurred in the reproductive tract of females in all five colonies of C. marginiventris. PV replication was limited to the portion of the calyx adjacent to the ovarioles. none