The bright yellow wall lichen, Xarzthoria parietina, is usually inhabited by oribatid mites (Acari) which do not only find shelter, but also graze on selected areas of the thallus. As X. parietina does not produce symbiotic vegetative propagules and its compatible photobiont, unicellular green algae of the genus Debouxia, are rare outside lichen thalli, we tested the hypothesis of dispersal of viable Trebouxia cells via acarine faeces. The lichenivorous mites, Trhypochtonius tectorum and Trichoribates trimaculatus, were isolated from thalli of X. parietina and cultured in the laboratory on a lichen diet. Light microscopic investigations of faecal pellets from mites that had been feeding on X . parietina indicated gut passage of intact ascospores and photobiont cells. In a series of experiments, viable algal and fungal cells contained in such faecal pellets were cultured. The taxonomic affiliation of these isolates was identified using molecular techniques, i.e. comparative investigations of nuclear ribosomal gene data (ITS 1 and 2, 5.8s rDNA) in the algal and fungal partners, and of the species-specific hydrophobin gene sequence in the fungal partner. Our culturing experiments demonstrated that the faecal pellets of both lichenivorous mites, upon feeding on X. parietina, contain viable ascospores and photobiont cells (Trebouxia arboricola) and thus might be a common and successful mode of vegetative short-and long-distance dispersal of this and numerous other lichen-forming ascomycetes and their photobionts. Future studies will have to elucidate the evolutionary significance of invertebrate interactions with lichens. 0 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 259-268.ADDITIONAL KEYWORDS: Acari -comparative rDNA sequence analysis culturing experiments -hydrophobin gene sequence -lichen-arthropod interactions -oribatid mites -Trhypochtonius tectorum -Trichoribates trimaculatus -vegetative dispersal of lichens.