Phage AR1 that specifically infects Escherichia coli O157:H7 has been characterized previously for its primary host contact molecule, the long tail fiber. To strengthen its attachment to the host bacteria, the phage may use its baseplate for a second contact. This study investigated the characteristics of AR1 baseplate by deducing the responsible genes including wac and genes 9 to 15. The wedge pin g11 and the short tail fiber g12 varied the most, whereas the others were highly conserved compared with their counterparts in T4. The degrees of gene conservation correlated well with the known functions of individual gene products.