Genetical work has shown that one locus and two alleles are involved in the inheritance of a seedcoat character of Spergula arvensis and that the hetero-zygous plants are intermediate between the two types of homozygous plants. It is thus possible to translate directly the observed geographical distribution of plant frequencies into distribution of gene frequencies. The inheritance of a hairiness character is less straightforward, but in this case also there is a close relation between phenotype frequency distribution and gene frequency distribution. Using the fact that plants heterozygous for the seedcoat character are easily recognizable, it is possible to make an estimate of the amount of outbreeding in nature. The values obtained range from o to 3 per cent.