Complete genome sequences have been determined for 73 tailed-phages that infect members of the bacterial Enterobacteriaceae family. Biological criteria such as genome size, gene organization and gene orientation were used to place these phages into categories. There are 13 such categories, some of which are themselves extremely diverse. The relationships between and within these categories are discussed with an emphasis on the head assembly genes. Although some of them are clearly homologues, suggesting a very ancient origin, there is little evidence for exchange of individual head genes between these phage categories. More recent horizontal exchange of phage tail fiber and early proteins between the categories occurs, but is probably not extremely rapid.