AbstractIn order to determine the localization of DNA‐synthesis inStreptomyces granaticolorandStreptomyces hygroscopicus, mycelia (growing either on agar or in liquid medium) were pulse‐labelled with3H‐thymidine and prepared for autoradiography. The distribution of silver grains showed no regions of preferential incorporation of3H‐thymidine in mycelia up 300 μm in length. Since mycelia grow by apical elongation of hyphae, the frequency of silver grains was quantitatively analysed along individual main hyphase. No significant difference of labelling was found within zones of different age up to a distance of 80 μm from the hyphal tip. Also, the very youngest part of the hyphae enclosing only the most apically situated nucleoid did not show any deviation from the average frequency of silv