Patterns of the molecular genetic diversity changes within groups of old and modern Bulgarian bread wheat varieties were analyzed using 19 wheat microsatellite markers and one secalin-specific sequence tagged site marker. A total of 91 genotypes representing almost the complete spectrum of varieties released in Bulgaria during the period 1925-2003 were included in the study. Analysis of microsatellite allele polymorphisms revealed quantitative (different allele frequency) and qualitative (loss of alleles and/or appearance of novel alleles) changes in genetic diversity over decades of variety release. Genetic diversity was high among both old and modern varieties with an average of 0.65. An average heterogeneity of 10.1 percent was established, this being highest in the old varieties. The genetic diversity was affected by the interaction between the breeding centre and the applied breeding strategy directed towards increasing the productivity or grain quality improvement.