The tail structure of the Salmonella phage P22 mediates both adsorption of the phage to its host and enzymatic hydrolysis of the bacterial O-antigen. The tail is an oligomeric structure, which is assembled from a single polypeptide species. We report here the amino- and carboxyl-terminal sequences of the P22 tail protein and the nucleotide sequence of its gene (gene 9). These data specify the complete amino acid sequence of the tail protein. The tail protein is a slightly acidic protein containing 666 amino acids. Comparison of the gene and protein sequences indicates that mature tail protein arises by cleavage of the initiator N-formyl-methionine from the nascent chain.