AbstractThe ovaries of endoparasitic species of braconid and ichneumonid wasps contain large numbers of polydnavirus (PDV) virions that replicate in specialized calyx cells of the ovaries and are injected into the host larva during parasitization. In the braconid waspCotesia congregatathat parasitizes the tobacco hornworm,Manduca sexta, the total amount of viral DNA present in the ovaries was determined to be 25–75 ng. Analysis of viral DNA on 0.4% agarose gels showed that the genome was comprised of 15–20 circles of double‐stranded DNA. SDS‐PAGE analyses showed that a large number (>30) of structural polypeptides were present in the virions, and analysis of the venom likewise showed that multiple components were present. The major size classes of venom proteins differed from those present in the PDV. However, Western blots using polyclonal PDV antibodies showed that some cross‐reacting PDV‐like proteins were present in the venom, perhaps explaining the mild PDV‐enhancing effect of the venom. Injection of PDV into unparasitized larvae provoked pronounced alterations in their growth, development, pigmentation, and hemolymph proteins. A densely staining band of hemolymph proteins of approximately 18–20 kD appeared in large amounts relative to other hemolymph proteins several days following injection of PDV; this band was undetectable in naturally parasitized larvae. Eggs which had been washed extensively to remove PDVs were encapsulated following injection, but development of the host still was disrupted, usually in the post‐wandering prepupal stage. Thus, neither the parasites nor their host survived, despite mobilization of an “effective” host response.