Growth kinetics, branch formation, and cytological properties of mycelial growth of Streptomyces hygroscopicus on solid media were investigated by phase-contrast microscopy using a microculture method. Measurements were made on taken photographs of the growing hyphae from the beginning of spore germination up to maximal 18 hours. The specific growth rate of the germ tube was much higher than the specific growth rate of the mycelium. The doubling time of the total length of the mycelial hyphae and the doubling time of branch formation was quite the same for the period investigated. After a short time of outgrowth each individual hypha grows at a constant rate, i.e. the length increases linearly, but the growth kinetics of the whole mycelium becomes exponentially by branching. It seems, that nucleoids only divide in that part of a hypha between the tip and the nearest branch. A cell unit (1.4-1.9 mum on different media) could be calculated by the length of a hypha and the number of nucleoids. A hypha is growing with the cell unit at the tip or with the polar cap only. No interkalary growth in length could be found. Branches were formed only up to about 100 mum from the tip on complex medium and up to about 50 mum from the tip on mineral salt medium. A simple method of mycelial growth has been developed. Some properties showing the connection between Streptomycetes and prokaryotic organisms on one hand and hyphal growing fungi on the other hand are discussed.