Five broad host range lipid-containing bacteriophages PRD1, PR3, PR4, PR5 and L17 isolated from different parts of the world were compared using biological and structural criteria. Virus morphology as well as genome sizes appeared to be identical. However, these viruses could be distinguished by restriction endonuclease mapping and by their structural protein patterns in SDS--gel electrophoresis. The viruses studied thus form a very close group of lipid-containing bacteriophages. We suggest PRD1 as a model organism for this group and that the group be called the PRD1 phage group.