Five phages, beta, gamma, 17, 112, and 156, isolated from lysed broth cultures and active on Streptomyces mediterranei were characterized with respect to serological inactivation, pH stability, and thermal inactivation rates. With the aid of five antisera, two serological groups, termed A and B and comprising the following phages, could be formed: group A, 156; group B, beta, gamma, 17, and 112. On the basis of serological similarities, the phages of group B are considered as mutants derived from an identical ancestor. Thermal inactivation rates and pH stability alone do not permit a clear-cut classification of the phages, but are useful as an aid in distinguishing among phages belonging to the same serological group.