Very long virus-like filaments were found in calyx region of the reproductive tracts of the braconid parasitoid, Microplitis mediator Haliday, which attacks the Noctuid, Pseudaletia ( = Leucania) separata Walker. These filaments are nuclear in origin; Feulgen and methyl-green pyronin reactions revealed cytochemically the presence of DNA. Filaments are attached to the surface of the chorin of an egg until hatching and function to suppress the encapsulation reaction of the host. Furthermore, it was observed that serosal cells, 18-h and 24-h from oviposition, released substances possibly related to the inhibition of encapsulation. none