Nonkiller progeny, lacking segments from the dsRNA genome of the virus associated with the P4 killer specifity, were recovered from a cross between a P4 killer strain and a sensitive strain. Three patterns of deletions were identified among the non-killers. In addition to the loss of killer activity these strains lost also the immunity and the ability to exclude the genomes of the virus associated with the P6 killer specifity but retained the essential information for viral coats. The patterns of deletions permitted the assignment of the killer function to 2 segments in the P4 genome, one in the medium group and the other in the lightest segment of the genome. Coat formation, as in the P6 virus, is associated with the heavy components of the dsRNA segmented genome but the information is organized somewhat differently from the organization of the virus associated with the P6 killer specifity. The loss of the exclusion function by the nonkillers enabled the reconstruction of "hybrid" viral genomes that restore specific killer activity. Thus, such hybrids indicate the position of the killer-related information in the P6 genome and suggest a role to the killer protein of P4 in the exclusion of specific dsRNA molecules.