Observations on living colonies of S. coelicolor strain A3(2) growing on cellophan films were made by phase-contrast micros-copy. Particular attention was paid to the mode of origin of the aerial mycelium. It was shown conclusively that aerial hyphae can originate as simple branches of the substrate mycelium, without any previous fusion of hyphae. The pattern of development of the substrate mycelium is markedly influenced by the composition of the medium.