Morphology and systematics of Trebouxia phycobionts in the thallus of Usnea longissima Key words: Lichens, Lecanorales, Usnea longissima, Chlorella[es, Trebouxia. - Morphology, ultrastructure, haustoria, phycobionts, systematics. Abstract: In the lichen genus Usnea different species of Trebouxia-phycobionts as well as different haustorial types are known. The isolated and cultivated phycobiont of Usnea longissima AcH. was studied by light- and electron microscopy and resembles in cytomorphological details the type of Trebouxia impressa AHMAD. In addition to simple wall-towall contacts between the symbiotic components also intraparietal (= intrawall-)haustoria could be observed as the normal interaction type.