Using mitochondrial DNA restriction endonuclease analysis, the dynamics of the natural Saccharomyces cerevisiae strains present in spontaneous wine fermentations have been studied. We observed a sequential substitution of Sacch. cerevisiae strains along fermentation agreeing with different fermentation phases. When the restriction pattern's similarity (measured as the fraction of shared restriction fragments) was high, a clear sequential substitution of the strains was seen. However, when the similarity was low, although a sequential substitution could be also observed between secondary strains, a clearly predominant strain was present along the whole fermentation process.