Ultrafine structure of the mycellium of the colonies of active and non-active variants of Actinomyces parvullus, producing actinomycin D was studied. Functional rebuilding of the cells of the substrate and aerial mycellium during the colony development in connection with synthesis and accumulation of the antibiotic by the culture was shown. The process of production and secretion of an amorphous material in the cells of the substrate mycellium of the active variant of Actinomyces parvullus was studied. It was supposed that the above process was associated with the antibiotic synthesis by the organism cells. The phenomenon of intrahyphal growth of the hyphae observed in both the active and inactive variants of the colonies was rather characteristic of the cells of the culture studied.