COLICINE H, which kills many Escherichia, Shigella, Salmonella and a few Proteus hauseri1 also attacks many strains of P. morganii2. The relationship between P. morganii and the other genera mentioned has recently been strengthened by the finding3 that the guanidine + cytosine content of their deoxyribonucleic acids are very similar and differ from P. hauseri, P. rettgeri and Providence strains. On the other hand, it has been shown4,5 that although P. morganii strains are not susceptible to P. rettgeri phages, many P. morganii organisms are attacked by P. hauseri and Providence phages. There is also a well-known antigenic relationship between Proteus morganii and P. hauseri. Work on P. morganii phages has been confined to temperate varieties, and host-range determinations were limited to the Proteus group of organisms2,6. The phages were found to be species specific. In view of the uncertainty7 about the taxonomic position of Proteus morganii, it was decided to investigate the action of P. morganii phages derived from both lysogenic strains and sewage on members of the family Enterobacteriaceae.