The genomic constitution of two S. cerevisiae baker's yeasts and their meiotic products have been analyzed by pulsed-field gel-electrophoresis, hybridization with specific gene probes, marker segregation, and flow cytometry. The parental strains have chromosomal patterns substantially different from those of laboratory strains used as controls. This pattern is partly the result of there being more than one copy of homologous chromosomes of different size, as judged by Southern-blot hybridization carried out with specific gene probes. Flow cytometry indicated that the strains have a 2.7 C DNA content. Tetrad analysis showed disomy for some chromosomes and tetrasomy for others. When two complete tetrads were subjected to molecular analysis the results confirmed instances of segregation of homologous chromosomes of different size. However, the presence of chromosomal bands absent in the parentals and the disappearance of chromosomal bands present in the parental strains were frequently seen. This result was attributed to two different phenomena: (1) the presence of multiple Ty1 and Ty2 transposable elements which seem to undergo interchromosomal translocation together with amplification, giving rise to differences in chromosomal size; (2) the presence of multiple Y' subtelomeric regions, giving rise to asymmetrical homologous recombination and, as a consequence, differences between the size of the recombinant chromosomes and the non-recombinant parental chromosomes. Chromosomal reorganization occurs with a very high frequency during meiosis. By contrast, mitosis is very stable, as judged by the reproducible electrophoretic karyotype shown by the parental strains in successive generations.