The nucleotide sequences of the left ends of bacteriophage Mu DNA and that of its close relative D108 have been determined. The first 100 bp of phages Mu and D108 are substantially the same except for an octanucleotide change from bp 53 to 61 and other small interspersed base-pair changes from bp 61 to 200. The first five host nucleotides preceding the host-phage junction are generally, but not always, G + C-rich and these five nucleotides display no obvious consensus sequence. Both phages Mu and D108 share striking similarity in their end DNA sequences to the end sequences of the newly described Escherichia coli movable genetic element IS30.