Mitosis, septation and branching were studied in undifferentiated mycelia and leading hyphae of Aspergillus nidulans, a mould which forms incomplete septa. After spore germination, nuclei divided synchronously until germ-tube hyphae contained 8 or 16 nuclei; mitosis occurred when the volume of cytoplasm per haploid nucleus was about 60 mum3. Intercompartment development was not synchronized, consequently mitosis in the mycelium as a whole eventually became asynchronous. During the stage of asynchronous compartment development, the nuclei, septa, branches and total length of undifferentiated mycelia all increased exponentially at approximately the same specific rate. Septa were formed in hyphae in groups of up to nine; the mean time required for the formation of a group of septa was about 9 min. The mean interval between successive cycles of septation in a hypha was approximately the same as the organism's doubling time. There was a highly significant correlation coefficient between septation and branch initiation and most intercalary compartments initially formed a single branch. The volume of cytoplasm per nucleus in a diploid strain was approximately double the value observed for a haploid strain. However, the length of the hyphal growth unit was not affected by ploidy. The study suggests that a duplication cycle can be recognized during mycelial growth which is analogous to the cell cycle observed in unicellular micro-organisms.