Sterile culture filtrates of Pythium sylvaticum inhibit the root growth of seedlings of flax and wheat. Just behind the root tip a pronounced thickening becomes visible, on which often many root hairs are formed. Growth of the other parts of the plants is hardly if at all influenced. Microscopical examination shows that the number of cortical cell layers in the affected roots is not increased as compared to that of normal roots, but cell growth is extended transversally, longitudinal growth being seriously restricted. The same symptoms can be induced by growing seedlings in a solution of indole-acetic acid.