Changes in genetic variability in populations (stand origins), full-sib (FS) families and three generations of selfed lines of Betula pendula were observed based on 15 allozyme loci. Growth vigour, measured as stem volume, and its relationship with heterozygosity was studied to determine the effect of inbreeding. Pooled FS families showed a higher percentage of polymorphic loci (P) and allelic numbers per locus (A) than those of natural populations, but no difference in heterozygosity. There was no difference in allozyme variability between fast-and slow-growing family groups, and heterozygosity was not correlated with stem volume among FS families. Allozyme variability was significantly decreased in advancing generations of selfing, and the further the selfing generation, the lower the heterozygosity and the slower the growth. Observed heterozygosity after advancing generations of inbreeding was increasingly higher than expected, indicating overdominance effects or, alternatively, selection against deleterious homozygotes.