AbstractStarting with germinating spores ofStreptomyces granaticolor, the kinetics of elongation of 3 types of outgrowing hyphae was determined. The 3 hyphal types were: first germ tube, second germ tube (which appeared from the same spores about 3 hours later) and branches arising from the first germ tubes. Taking photographs every fifth minute, with all hyphae a linear multiphasic elongation behaviour was found. Alternatively, periods with constant elongation rate (α) changed with steps at which α increased abruptly. Several successive shifts were observed until finally at a hyphal length of about 25 μm a constant maximum α of about 22 μm h−1was attained.The length of the periods during which α remained constant did not depend on either the hyphal type or the order of the period. On an average, a single period lasted 46.8 minutes.The lengths of the hyphae, at the time when α increased, corresponded for the first germ tubes to successively about 2,4,8 and 16 unit cells. With branches and second germ tubes these lengths corresponded to 4 and 10 unit cells.Concerning increase of α during each step the quotients were in the range between 1.3 and 2.0. Branches and second germ tubes started with twice the rate observed with first germ tubes and thus reached the maximum α after a reduced number of steps. Calculations indicated, that on principle α of each linear period was determined by the number of unit cells (nucleoids) present at it