Campoletis sonorensis virus (CsV) (Polydnaviridae) previously was detected only in the calyx epithelial cells and lumen of the oviducts from female C. sonorensis (Ichneumonidae) endoparasitic wasps (Norton et al., Cell Tissue Res. 162:195-208, 1975). Using dot-blot hybridizations, we detected low amounts of CsV DNA in male and female wasp head and thorax tissues and in male abdominal tissues. Low amounts of extrachromosomal viral DNA were detected in Southern blots of undigested male wasp DNA and in male DNA purified by isopycnic centrifugation. High-molecular-weight male wasp DNA digested with any of several restriction endonucleases and hybridized with cloned viral DNAs from CsV superhelices B and Q under stringent conditions contained CsV-specific DNA fragments that differed significantly in size and number from the hybridizing fragments detected in comparably digested viral DNA. Identical offsize restriction fragments were detected in digested female head and thorax DNA. These data suggest that at least CsV DNAs B and Q are integrated in C. sonorensis cellular DNA and that the virus may be transmitted through the germline.