The DNA of bacteriophage Mu has been studied to characterize a region of inhomogeneous sequence that occurs at one end of the molecule. The kinetics of reassocation of tracer amounts of labeled host DNA in the presence of Mu DNA show that Mu DNA contains a complete selection of host sequences. These host sequences are shown to be covalently attached to phage-specific sequences and are present at a concentration that accounts for the inhomogeneity observed in the electron microscope. The significance and possible function of the host DNA attachment is discussed.