AbstractDiploid monoeciousPellia(n= 18) in Britain coincides cytologically with material from Belgium, Poland and the U.S.S.R. Each of its chromosomes is identified by Giemsa C-banding as unique and differs from those of haploid species. Despite resemblingP. epiphylla(L.) Corda (n= 9) in gametophytic and sporophytic characters, the haploid and diploid karyotypes remain distinct throughout overlapping geographical distributions and show no cytological evidence of gene exchange. They justify specific recognition which, in the absence of discontinuity of morphological variation, requires the provision of an infallible means of discrimination. A straightforward cytological procedure is supplied for the purpose andP. borealisLorbeer is described as new to Britain.