The biology of Inocutis dryophila (Berk.) Fiasson & Niemelä was studied on Quercus robur L. in Lithuania, based on 22 host trees. Decay columns caused by the fungus in six oaks,120180 years old, were examined in detail by dissecting the stems. Length of the decay columns in different trees varied between 4.8 and 11 m. Vegetative compatibility tests with the isolates indicated that each rot column comprised of a single genet of the fungus, and that the rot columns in different trees represented different genets. Basidiocarps of the fungus were recorded from 0.7 to 12.5 m above the ground. Infection routes, forestry, and biodiversity aspects are discussed. Colour photographs are shown of the basidiocarp, decay of different stages, and confrontation tests among cultivated mycelia of the same and different genets.