SUMMARY : Electron microscope observations were made on submerged cultures in defined medium of two species of the genus Streptomyces. The evidence obtained con- firms the supposition of Klieneberger-Nobel (1947), with surface cultures of four other species, that at an early stagesin the life history of the organism concerned, fusion occurs between portions of the same or different hyphae, culminating in the formation of 'initial cells'. In S. griseus initial cells were observed at 26 hr. and as late as 6 days. Certain unidentified bodies were observed. These may be related to initial cell formation. It has been known for some time that species of the genus Streptomyces show two different growth phases-the substratum mycelium and the aerial mycelium (0rskov, 1923). These phases were renamed the primary and the secondary mycelium, respectively, by Kleineberger-Nobel ( 1947) ; her termino- logy will be used in this paper. The recognition of diphasic growth, along with a persistent branching filamentous habit and reproduction from spores, suggests, according to Bisset & Moore (1949), a clear separation from the other Actinomycetales (Bergey's Manual), and on this basis the 'four sporing Actinomycetes ' described by Klieneberger-Nobel in her paper may be con- sidered as species of the genus Streptomyces. This author made the first critical examination of the life cycle of these organisms as it occurs in surface cultures on a solid agar medium or on a liquid medium. By combining a staining method based upon that of Robinow (1942) with careful micro- scopical examination, she found a marked difference in the two phases of growth in the species examined. The primary mycelium, developed from germinating spores, is septate and profusely branching, and the individual hyphae, which are somewhat less than 1 p. in diameter, contain 'granular or rod-shaped chromatin bodies '. The secondary spore-bearing mycelium, as she describes it, arises directly from the primary mycelium in agglomerations of the latter and from points of fusion which Klieneberger-Nobel terms ' initial cells'; it is distinguished in that it is thicker, branches less profusely, eventually breaks into single cells or spores and has a more clearly defined internal structure. According to Klieneberger-Nobel the spores are developed after fusion and organization of the cellular material, processes whose exact signifi- cance is not yet apparent. Klieneberger-Nobel suggested that the complete cycle might represent alternating generations of haploid primary mycelium with diploid secondary mycelium, with sexual fusion occurring in the initial cell, but stated that 'in order to prove this an analysis of "chromosome"