gpW is a 68 residue protein known to be present in phage particles. Extracts of phage-infected cells lacking gpW contain DNA-filled heads, and active tails, but no infectious virions. gpW is required for the addition of gpFII to the head, which is, in turn, required for the attachment of tails. Since gpFII and tails are known to be attached at the connector, gpW is also likely to assemble at this site. The addition of gpW to filled heads increases the DNase resistance of the packaged DNA, suggesting that gpW either forms a plug at the connector to prevent ejection of the DNA, or binds directly to the DNA. The large number of positively charged residues in gpW (its calculated pI is 10.8) is consistent with a role in DNA interaction.