The three dsRNA genomic segments of bacteriophage Φ13 were copied as cDNA and the nucleotide sequences were determined. The organization of the genome is similar to that of Φ6, and there is significant similarity in the amino acid sequences of the proteins of the polymerase complex and one of the membrane proteins, P6. There is little or no similarity in the nucleotide sequences. Several features of the viral proteins differ markedly from those of Φ6. Although both phages are covered by a lipid-containing membrane, the protein compositions are different. The host attachment protein consists of two peptides rather than one and the phage attaches directly to the LPS of the host rather than to a Type IV pilus. Despite the differences in the structure of the membranes, the two viruses can successfully exchange the genes for host attachment proteins and thereby change their host specificities.