The genome sequence of bacteriophage phiA1122 has been determined. phiA1122 grows on almost all isolates of Yersinia pestis and is used by the Centers for Disease Control and Prevention as a diagnostic agent for the causative agent of plague. phiA1122 is very closely related to coliphage T7; the two genomes are colinear, and the genome-wide level of nucleotide identity is about 89%. However, a quarter of the phiA1122 genome, one that includes about half of the morphogenetic and maturation functions, is significantly more closely related to coliphage T3 than to T7. It is proposed that the yersiniophage phiA1122 recombined with a close relative of the Y. enterocolitica phage phiYeO3-12 to yield progeny phages, one of which became the classic T3 coliphage of Demerec and Fano (M. Demerec and U. Fano, Genetics 30:119-136, 1945).