Background: The T4 bacteriophage consists of a head, filled with double- stranded DNA, and a complex contractile tail required for the ejection of the viral genome into the Escherichia coil host. The tail has a baseplate to which are attached six long and six short tail fibers. These fibers are the sensing devices for recognizing the host. When activated by attachment to cell receptors, the fibers cause a conformational transition in the baseplate and subsequently in the tail sheath, which initiates DNA ejection. The baseplate is a multisubunit complex of proteins encoded by 15 genes. Gene product 9 (gp9) is the protein that connects the long tail fibers to the baseplate and triggers the tail contraction after virus attachment to a host cell. Results: The crystal structure of recombinant gp9, determined to 2.3 Å resolution, shows that the protein of 288 amino acid residues assembles as a homotrimer. The monomer consists of three domains: theN-terminal domain generates a triple coiled coil; the middle domain is a mixed, seven-stranded β sandwich with a topology not previously observed; and the C-terminal domain is an eight-stranded, antiparallel β sandwich having some resemblance to 'jelly-roll' viral capsid protein structures. Conclusions: The biologically active form of gp9 is a trimer. The protein contains flexible interdomain hinges, which are presumably required to facilitate signal transmission between the long tail fibers and the baseplate. Structural and genetic analyses show that the C-terminal domain is bound to the baseplate, and the N-terminal coiled-coil domain is associated with the long tail fibers.