Cladina subtenuis is the most commonly found ‘ reindeer lichen ’ in the southeastern United States. In the present study C. subtenuis was examined for polymorphism in the small subunit (SSU) ribosomal DNA (rDNA) within and among small clumps of podetia, here referred to as mats, from five geographical locations. Polymorphism in the SSU rDNA was not detected among podetia within mats, but polymorphism was detected among mats from a single location. Therefore, mats of C. subtenuis may represent a single genetic individual, although genetically distinct mats may occur in close proximity. Their proximity couldencourage sexual reproduction between genetically distinct individuals, which would maintain variation in these populations. The SSU rDNA types represented three different lengths and restriction-site patterns, reflecting the presence of sequence insertions of at least 200 nucleotides in the SSU rDNA coding region. Thei nsertions, differing in position, number and sequence, are group I introns similar to those previously reported in the Cladonia chlorophaea complex. The intron variation among the SSU rDNA types indicates that molecular evolution has occurred within this species, perhaps since its divergence from related taxa and dispersal across the southeastern United States.