Cementing proteins that bind to the virion surface have been described in double-stranded DNA viruses such as herpesvirus, adenovirus, and numerous bacteriophages. The three-dimensional structure of bacteriophage L determined by electron cryo-microscopy reveals binding modes of two cementing proteins-one, called Dec, encoded by phage gene orf134 and the other by an as yet unidentified gene. These two proteins form homotrimers and bind at the quasi 3-fold axes nearest the icosahedral 2-fold axes and at the icosahedral 3-fold vertices, respectively. They do not bind at the quasi 3-fold axes near the icosahedral 5-fold vertices. These observations indicate precise recognition of the two cementing proteins at a subset of the quasi equivalent sites on the phage capsid. Sequence analysis shows striking similarity between the C-terminal portion of phage L Dec protein and five regions in the long tail fiber of a T4-like phage, suggesting functional parallelism between them.