We have analyzed the evolution of chromosome maps of Hemiascomycetes by comparing gene order and orientation of the 13 yeast species partially sequenced in this program with the genome map ofSaccharomyces cerevisiae. From the analysis of nearly 8000 situations in which two distinct genes having homologs inS. cerevisiaecould be identified on the sequenced inserts of another yeast species, we have quantified the loss of synteny, the frequency of single gene deletion and the occurrence of gene inversion. Traces of ancestral duplications in the genome ofS. cerevisiaecould be identified from the comparison with the other species that do not entirely coincide with those identified from the comparison ofS. cerevisiaewith itself. From such duplications and from the correlation observed between gene inversion and loss of synteny, a model is proposed for the molecular evolution of Hemiascomycetes. This model, which can possibly be extended to other eukaryotes, is based on the reiteration of events of duplication of chromosome segments, creating transient merodiploids that are subsequently resolved by single gene deletion events.