The identity of photobionts from 20 species of the Physciaceae from different habitats and geographical regions has been determined by ITS rDNA sequence comparisons in order to estimate the diversity of photobionts within that lichen group, to detect patterns of specificity of mycobionts towards their photobionts and as a part of an ongoing study to investigate possible parallel cladogenesis of both symbionts. Algal-specific PCR primers have been used to determine the ITS rDNA sequences from DNA extractions of dried lichens that were up to 5 years old. Direct comparisons and phylogenetic analyses allowed the assignment of Physciaceae photobionts to four distinct clades in the photobiont ITS rDNA phylogeny. The results indicate a diversity within the genus Trebouxia Puymaly and Physciaceae photobionts that is higher than expected on the basis morphology alone. Physciaceae photobionts belonged to 12 different ITS lineages of which nine could unambiguously be assigned to six morphospecies of Trebouxia. The identity of the remaining three sequences was not clarified; they may represent new species. Specificity at the generic level was low as a whole range of photobiont species were found within a genus of Physciaceae and different ranges were detected. The photobionts of Physcia (Schreb.) Michaux were closely related and represented one morphospecies of Trebouxia, whereas the algal partners of Buellia De Not and Rinodina (Ach.) Gray were in distant lineages of the ITS phylogeny and from several Trebouxia morphospecies. Photobiont variation within a genus of Physciaceae may be due to phylogeny, geographical distance or because photobionts from neighbouring lichens were taken (“algal sharing“). At the species level Physciaceae mycobionts seem to be rather selective and contained photobionts that were very closely related within one morphospecies of Trebouxia.