Heterogeneity in polydnavirus DNA was exploited as a means of following the transmission of viral genomes in parasitoid populations. Paarasitoid lines isogenic for viral DNA markers were established from both a braconid (Cotesia melanoscela) an an ichneumonid (Hyposoter fugivitus) species. In crossing experiments these markers routinely segregated in Mendelian (chromosomal) fashion, suggesting that the structure of polydnavirus genomes is probably determined by the integrated form of viral DNA, rather than by extrachromosomal molecules. none