InStaphylococcus aureusstrains of type 83A/6557, and in “type 6557” and other group III strains insusceptible to phage 83A, two kinds of temperate phages were frequently found, belonging to the serological group A (α phages) and B (β phages), respectively. The two phages have been characterized by conventional criteria including plaque morphology, serological grouping, host range, and ultrastructure. To meet with the need for more specific identification the following characteristics were included: lysogenic conversion—in casuthe ability to convert a positive Tween 80 reaction to a negative one—, ability to block susceptibility to certain typing phages, and kinetics of inactivation with specific anti‐phage sera. These tests were found valuable and may deserve wider application as criteria in phage identification. It is made probable that the β phage has been responsible for the evolution from type 83A/6557 to “type 6557”, due to its ability to block susceptibility to phage 83A. Suggestive evidence is presented for an actual “epidemic” spread of this particular phage within the existing population of susceptible hospital staphylococci, contributing materially to the recently observed change in prev