The 121,752-bp genome sequence of bacteriophage T5 was determined; the linear, double-stranded DNA is nicked in one of the strands and has large direct terminal repeats of 10,139 bp (8.3%) at both ends. The genome structure is consistently arranged according to its lytic life cycle. Of the 168 potential open reading frames (ORFs), 61 were annotated; these annotated ORFs are mainly enzymes involved in phage DNA replication, repair, and nucleotide metabolism. At least five endonucleases that believed to help inducing nicks in T5 genomic DNA, and a DNA ligase gene was found to be split into two separate ORFs. Analysis of T5 early promoters suggests a probable motif AAA{3, 4 T}nTTGCTT{17, 18 n}TATAATA{12, 13 W}{10 R} for strong promoters that may strengthen the step modification of host RNA polymerase, and thus control transcription of phage DNA. The distinct protein domain profile and a mosaic genome structure suggest an origin from the common genetic pool.