AbstractExtensive cytological variation between British populations ofPellia endiviifolia(Dicks.) Dum. (n = 8+x/y) permits the recognition of six distinct karyotypes; one of them found also in Greece and another in the U.S.S.R. Although based on typically small amounts of constitutive heterochromatin, Giemsa C-banding patterns serve to identify each of three chromosomes, 1, 6 and 9, as dimorphic or polymorphic. Relative proportions of chromosomes differ between karyotypes and further variation relates toa nucleolar organiser region (NOR) on chromosome 6 of one population in addition to the NOR generally located in chromosome 7. These findings have repercussions intaxonomy and reproductive behaviour that are discussed in detail.