The structure of the purified lipid-containing phage PR4 was studied electron microscopically using thin sectioning, negative staining and freeze-fracturing techniques. The lipid layer was located inside a rigid capsid and had a bilayer structure. The innermost dense core was probably composed solely of DNA since no major structural proteins were missing in empty phage particles lacking DNA. During infection, phages were attached to the cell wall of the host. They apparently inject their DNA through the cell envelope. The lipid layer of the phage might play an active role in the injection process. The maturation of the phage capsid takes place within the nuclear region of the cell, whereas intact phages with DNA were always seen in the cell periphery.