Sharp discontinuities in chromosome number and type of breeding system provide more satisfactory characters for classifying the members of the Linum perenne group than most of the morphological characters which have been used in the past. Pollen morphology has great taxonomic value here because it can be used to distinguish diploids from tetraploids and inbreeders from outbreeders. Of seventy European topodemes examined, fifty-seven are diploid (2n= 18) and fourteen are tetraploid (2n= 36). The pollen grains of the diploids have three furrows and those of the tetraploids have six pores. All of the European members of the group are heterostylous and self-sterile, except for L. leonii, which is homostylous and self-fertile and endemic to Erance and Western Germany. Artificial hybrids between most of the European members of the group have been produced and their pollen fertilities recorded. No absolute sterility barriers have been found and the whole group is equivalent to a single coenospecies. The course of evolution in the group is considered in the light of all the available evidence.