Fragmentation rates in 21 populations of three foliose lichens Xanthoparmelia conspersa, Melanelixia fuliginosa ssp. fuliginosa and Parmelia saxatilis and the crustose lichen Rhizocarpon geographicum were estimated in north Wales, UK from the size frequency distribution of thalli with fragmenting centres. The Kaplan-Meier estimator suggested significant differences in fragmentation rates among populations of three of the four species. In M. fuliginosa ssp. fuliginosa, Cox regression analysis also suggested higher rates of fragmentation on rock surfaces with a greater percent lichen cover and increased diversity (Shannon-Weaver diversity index). Slope angle, aspect, texture, and location of population relative to the sea also influenced fragmentation rates. The data suggest considerable variation in fragmentation rates among populations within the same area which may be determined primarily by the intensity of competition on a rock surface.