Molecular sequence data of the nuclear ITS region was used to investigate the diversity of photobionts in Polish samples of Flavocetraria nivalis. The samples came both from alpine habitats, as well as from lowland localities near the coast. All green algal symbionts were identified as members of the Trebouxia simplex aggregate. These were compared with those of additional samples from Flavocetraria nivalis collected in different parts of Europe and also with photobionts assigned to T. simplex from other lichens. Within the T. simplex aggregate, the Trebouxia ITS sequences from F. nivalis formed four clades. In the Polish lowland populations only a single clade of T. simplex was detected which also occurs in Polish mountains, south Sweden and Austria. A further clade of T. simplex is present in F. nivalis from Polish mountains and is also known from F. nivalis further north in Scandinavia and Greenland, as well as from other lichens in Sweden, the Austrian Alps, and Antarctica.