The Psu (Polarity suppression) protein of satellite bacteriophage P4 was first characterized as an anti-terminator of transcription termination in Escherichia coli. Psu is also a structural component of mature P4 capsids, where it is present as a decoration protein. Psu is located externally on the capsid surface, and it appears to protect the capsid from loss of DNA through the capsid shell. The ability of Psu to specifically bind to the P4 capsid appears not to be dependent on any P4 specific components such as the capsid protein cleavage products h1 and h2, or P4 DNA. We suggest that Psu binds to the P4 capsid as a result of the special structure of the hexamers in the P4 capsid.