The lichen Diploschistes muscorum (Scop.)R.Sant. starts its thallus development as a parasite of the squamules of different Cladonia species, but fully developed Diploschistes thalli live independently. D. muscorum develops apothecia in the Cladonia squamules and is associated at that stage with the phycobiont of Cladonia, Trebouxia irregularis. Augmentation of the Cladonia phycobiont and growth of the D. muscorum hyphae result in a development of young Diploschistes thalli from the Cladonia squamules. As the Cladonia increasingly disintegrates, D. muscorum becomes independent. In fully developed thalli the Cladonia phycobiont can be exchanged for T. showmanii, an alga different in morphology and reproduction from T. irregularis. In one case, a fully developed D. muscorum thallus with both phycobionts was demonstrated. A fully established thallus can also be formed by the growth of small thallus swellings, which cover Cladonia squamules.