Intraspecific thallus interactions in mixed populations of Rhizocarpon lecanorinum and R. geographicum in North Wales are described. In contrast to the most commonly occurring morphotype of R. geographicum, which forms mosaics comprised of sharply delimited individual thalli, R. lecanorinum thalli consistently merge with one another at intraspecific contacts, leaving no visible trace of their initial boundaries. On the analogy of mycelial interactions in populations of wood-rotting and other non-lichen-forming fungi, it is postulated that these differences are the outcome, in R. geographicum, of somatic incompatibility between distinct genotypes, and in R. lecanorinum of somatic compatibility among genetically identical or very closely related thalli. Possible mechanisms for the evolution of clonal population structure in R. lecanorinum and links between somatic interactions and reproductive output in these species are discussed. Facultative parasitism of R. lecanorinum by Schaereria fuscocinerea, and resistance to such parasitism by cooccurring R. geographicum, are also documented.