The system of hyphal branching by Merulius lacrymans was observed in mycelium which had grown from a wood food-base on to glass slides during incubation in sterile moist chambers. A hierarchy of branches and sub-branches arose from the region of clamp connexions, or nodes, of relatively wide main hyphae. There was evidence that the sequence of branches occurring at nodes in basipetal succession represented the time sequence of branch development at any one node. Later-formed branches at any node were smaller than earlier branches, but such earlier branches usually became smaller towards the tip as growth continued. Mycelial strands were built up by growth and branching of thigmo-tropically sensitive ‘tendril’ hyphae in association with the wide main hyphae. Tendril hyphae were characteristically narrow, thin-walled hyphae arising both as later-formed branches from the nodes of the main hyphae and as the narrowed tips of earlier branches. Although this branching behaviour could be seen amongst aerial hyphae growing over agar media, hyphae growing in contact with or within the agar behaved differently and did not form strands.