A population of Pellia borealis, n = 18, from Diabli Skok reservation (north west Poland) is polymorphic with respect to two peroxidase alleles PX 1b and PX 2" (63 and 37 per cent respectively), the spatial distributions of which are non-random. Thalli growing in a close neighbourhood exhibit only one of the two mutually excluding alleles, as shown by analysis of 54 samples, thus pointing to intense vegetative reproduction of thalli within individual samples. The clonal structure extends beyond the area of the sample and frequently includes the whole carpet of P. borealis. Apart from intense vegetative reproduction, differential selective value of thalli may constitute another factor shaping clonal structure of samples and carpets.Thirty-eight out of 40 analysed progenies were found to be monomorphic with respect to PX I" and PX 2" alleles. Sporangia from which these progenies were obtained have been collected from monomorphic samples of corresponding PX 1" and PX 2" phenotypes. The results are interpreted as reflecting seif-fertilisation and contrast with earlier data on high frequency of cross-fertilisation noted in P. epiphylla, n =9.