Fungi are one of the most diverse carbon source-assimilating organisms, living as saprobes, parasites and symbionts; they play an important role in carbon cycling in the ecosystem. A lichen thallus provides habitats for many non-lichenized fungi and usually contains large quantities of polyols. However, research has not been undertaken to identify carbon sources of lichen-inhabiting fungi. In this study, we isolated various lichen-inhabiting fungi from surface-sterilized Ramalina spp., Flavoparmelia caperata and Peltigera degenii, and demonstrated their ability to assimilate carbon sources, namely glucose, ribitol and mannitol. Several isolates efficiently assimilated mannitol and ribitol; however, most isolates could assimilate only mannitol or both ribitol and mannitol at low levels. It is suggested that there are different preferences and niche segregation of carbon sources among lichen-inhabiting fungi, and that this assemblage includes fungi with different lifestyles such as saprobes, endophytes and transient visitors.