SUMMARY: The life cycle of a Streptomyces sp. was studied by the Robinow HCl-Giemsa method of nuclear staining, and is described in the following manner: (1) initial nuclear division phase; (2) primary mycelium; (3) secondary mycelium (including aerial); (4) the formation of spores. It was observed that the method of nuclear division in the ungerminated spores closely resembled mitosis of certain higher organisms and lower plants, by the appearance of two terminal bodies whose behaviour was similar to mitotic centrosomes. The primary mycelium developed after the initial nuclear division phase and produced side branches, then later gave rise to single swellings in the hyphae. These swellings grew to form large round cells, each of which contained many nuclei. It was from these cells that the secondary mycelium developed, a part of which became aerial and terminated in the form of chains of uninucleate spores.