SUMMARY: The various stages of the reproductive cycle in freshly transplanted cultures of avian tubercle bacilli were recorded by a series of successive electron micrographs. An early stage of cytoplasmic differentiation is described in which discrete cellular units form inside the elongating filaments. Their significance and relation to the vacuolization seen at later stages is discussed. Evidence is produced to show that the dense bodies of mycobacteria are not nuclei. Free granules do not appear to play any part in the reproductive cycle in the normal type of development. In mycelial variants lysis is often seen to initiate new growth which starts from submicroscopical elements and micro-rods. The electron microscope has already produced valuable information about the internal structure of tubercle bacilli in various stages of development. How- ever, the results obtained still lack coherence and have given rise to con- flicting interpretations of the cytological details observed, In particular there has been controversy concerning the nature of the dense spherical bodies which seem to be a characteristic of the species of Mycobacterium although they are also present in related species (Mudd, Brodie, Winterscheid, Hartman, Beutner & McLean, 1951). They were first described by Lembke & Ruska (1940) and have been variously identified as nuclei (Knaysi, Hillier & Fabri- cant, 1950), nucleoids or karyoids (Ruska, Bringmann, Neckel & Schuster, 1952), mitochondria (Mudd, Winterscheid, DeLamater & Henderson, 1951), metachromatic granules (Werner, 1951) and skeletal structures for the support of nucleic acids, phosphates, lipoids, waxes, etc. (Lembke, 1947). Again there is considerable difference of opinion as to the nature of the intracellular organi- zation seen at various stages of development (Brieger, Crowe & Cosslett, 1947; Brieger & Cosslett, 1949; Knaysi et al. 1950; Ruska et al. 1952). The correlation of morphological and intracellular changes with stages in the reproductive process requires an almost continuous series of electron micrographs covering every phase of the development. The present paper describes results obtained by paying particular attention to the early stages of development of various avian strains grown on electron microscope grids placed on a variety of media.