Members of the ØX174 group of coliphages are all morphologically identical. The host range patterns of the principal isolates on strains of Escherichia coli are shown to form three distinct groups. These are found to coincide with three serological groups. Some basic properties of the isolate St-1, which is the only one able to grow on male strains of E. coli, are described. Sections of phage-infected cells were examined in the electron microscope, and the structural changes produced by ØX174 and St-1 were found to be similar. The commonest mechanism of lysis consists of the formation of a small bulge at the midpoint of a dividing cell; this bursts and releases the cell contents.