The global bacteriophage population is large, dynamic, old, and highly diverse genetically. Many phages are tailed and contain double-stranded DNA, but these remain poorly characterized genomically. A collection of over 1,000 phages infecting reveals the diversity of phages of a common bacterial host, but their relationships to phages of phylogenetically proximal hosts are not known. Comparative sequence analysis of 79 phages isolated on shows these also to be diverse and that the phages can be grouped into 14 clusters of related genomes, with an additional 14 phages that are "singletons" with no closely related genomes. One group of six phages is closely related to Cluster A mycobacteriophages, but the other phages are distant relatives and share only 10% of their genes with the mycobacteriophages. The phage genomes vary in genome length (17.1 to 103.4 kb), percentage of GC content (47 to 68.8%), and genome architecture and contain a variety of features not seen in other phage genomes. Like the mycobacteriophages, the highly mosaic phages demonstrate a spectrum of genetic relationships. We show this is a general property of bacteriophages and suggest that any barriers to genetic exchange are soft and readily violable. Despite the numerical dominance of bacteriophages in the biosphere, there is a dearth of complete genomic sequences. Current genomic information reveals that phages are highly diverse genomically and have mosaic architectures formed by extensive horizontal genetic exchange. Comparative analysis of 79 phages of shows them to not only be highly diverse, but to present a spectrum of relatedness. Most are distantly related to phages of the phylogenetically proximal host , although one group of phages is more closely related to mycobacteriophages than to the other phages. Phage genome sequence space remains largely unexplored, but further isolation and genomic comparison of phages targeted at related groups of hosts promise to reveal pathways of bacteriophage evolution.