SUMMARY.A brief account is given of the literature on zone lines in wood, and a preliminary attempt is made to classify the zone lines into groups in order to provide a basis for systematic research and to facilitate the statement of results. A single fungus,X. polymorpha, suspected of causing black lines in hardwoods, is dealt with morphologically and culturally. The development and stromatal production of the fungus on artificial media and wood blocks are described.X. polymorphais established as a black line producing fungus by its production of black lines in inoculated wood blocks. The morphology of the black lines is described in detail and their actual formation has been observed in cotton‐wool. As a result the Suggestion is made that the black lines are the marginal zones of entostro‐mata in the substratum comparable to those occurring in theDiaporthe.The black lines produced by the generaNummularia, Ustulina, HypoxylonandDaldiniain the substratum are stated to be of similar structure and significance.An account is given of aX. polymorphablack line superimposed upon the zone line formed by the attack ofFomes applanatuson beech wood. The confusion existing in the literature on the zone lines ofF. applanatusis partly attributable to this not uncommon phenomenon. The value of zone lines as criteria in the diagnosis of wood rots is discussed and some of the difficulties in the identification of a zone line in a typical rot are pointed