Plate (or slope) cultures of cerulomycin-producing Streptomyces coerulescens (KCC S-0360) showd spontaneously develping pocks which increased in number during subculturings. No spore formation and reduced aerial hyphae formation were observed in the pocks, whereas the formation of substrate hyphae were not inhibited. Almost all of the hyphae were broken or lysed in the pocks, and many phage tail-like particles, containing long tail type and bullet-shaped type, were observed in the pocks. The bullet-Shaped particles often formed a hexagonal crystal. No activity of self-replication was associated with the particles. The production or these particles did not occur in the liquid culture and the young or normal plate culture having no pocks. These results were similar to those of thiostrepton-producing Streptomyces azureus (ATCC 14921) and endomycin-producing Streptomyces endus (KCC S-0213), which have been described previously.