In a beech population composed of three parts (demes), the genetic structure of the seed (after dispersal) of each deme was recorded in two successive years and at five protein loci, four of which were polymorphic. Spatial and temporal differentiation, as evidenced in allele frequencies, genic diversity and heterozygosity, was studied. While no differentiation was found for diversity, allele frequencies and heterozygosity showed significant spatial differentiation at the genically most diverse loci in both years. Temporal differentiation measured as genetic differences between the seed production of the two years was distinctly lower. The total seed production over both years showed a tendency of homogenising spatial genetic differences present in each single year. This observation can be explained by local differences in flowering of the trees from one year to the next combined with non-random dispersal of the seed, where, however, the reproducing parts of the stand were genetically similar in both years.Comparisons with the results of related studies on ponderosa pine revealed remarkable similarities in the amounts of spatial subpopulation differentiation. Noticeable differences occur with respect to the individual deme levels of differentiation. Consequences for the interpretation of adaptive strategies of trees are briefly discussed.The data on diversities and conditional heterozygosities suggest that there is a tendency for rare alleles to occur preferentially in homozygous form among the seed. The significance of this phenomenon for the maintenance of genetic polymorphisms is demonstrated in the light of recent population genetic selection theories.