An exceptional group-I intron-like insertion at position 940 of the nuclear small subunit rDNA is found in lichen mycobionts of the families Parmeliaceae and Lecanoraceae. This shared insertion site is exceptional as it follows a G. Although several features of the self-splicing Tetrahymena intron are missing, the conserved structure of the presumed core region indicates that the new intron-like insertion, which is missing in mature transcripts, is not part of a silenced ribosomal repeat. It is unlikely that the new insertion is horizontally transferred from the adjacent position 943. A phylogenetic analysis indicates congruence with lichen phylogeny and suggests that the insertion has been vertically inherited.