The cohesive termini including the cos region (altogether 414 bp) of the DNA of the temperate coliphage N15 are sequenced. The termini are complementary 12-nucleotide single-stranded 5'-extended DNAs. The sequence of the left terminus is 5'-GGGCGGCGTCCG-3', that of the right 5'CGGACGCCGCCC-3'. Ten nucleotides of the N15 termini are identical to those of phage lambda. The N15 and lambda sequences are notably homologous only within the 50 bp region from the left and right ends. Phage N15 has a region with the nucleotide sequence identical to the R4 site of phage lambda, presumably reacting with terminase. This region is situated in the same site with regard to the cohesive sequence as in phage lambda. The cos region of N15 has no sequences similar to R1, R2, and R3 of lambda. N15 has a sequence similar to IHF of phage lambda, but in N15 this sequence is located near the right but not left (as in phage lambda) terminus. Computer analysis revealed palindromes and repeats within 450 bp of N15, including the cohesive termini.