The T4-type of bacteriophages is broadly defined on the basis of parti
cle morphology. It occurs in enterobacteria (125 representatives), aci
netobacters, aeromonads, pseudomonads, and vibrios (16 isolates). In a
ddition, 18 apparently unrelated phages with prolate heads and contrac
tile tails are found in a wide range of bacteria. A descriptive catalo
gue of these phages is presented. The T4-type probably originated in p
recursors of enterobacteria.