In a comprehensive study of the microflora of larch cankers, it was shown that Trichoscyphella willkommii (Hart.) Nannf. was the only fungus consistently associated with canker extension tissue. A total of eighty-three other fungal species was isolated from 208 cankers and a sequential colonization of cankers by these fungi was demonstrated. The spread within the canker of other species coincided with disappearance of T. willkommii. The four most frequently isolated canker invading fungi, Cryptosporiopsis abietina, Zalerion arboricola, Tympanis laricina and a Phialophora sp. were shown to be able to replace T. willkommii in bark blocks, while Z. arboricola and Phialophora sp. brought about the death of its hyphae on agar plates. Inoculation of the four species either into branch lesions in which T. willkommii was established, or jointly with T. willkommii into new incisions, showed that lesion extension was markedly decreased in their presence. It was suggested that the natural healing of larch cankers is largely initiated because T. wilkommii is placed at an ecological disadvantage through having to compete with other fungi.