Analysis of mitochondrial DNA restriction patterns was used to study the introduction of a selected strain of Saccharomyces cerevisiae for fermentation of non-sterile musts of La Rioja (Spain). All of the isolates from the inoculated musts showed the restriction pattern of the selected strain. The same technique was used to study the spontaneous fermentation of musts, showing that a few strains were responsible for the fermentations. One of the strains identified from the spontaneous fermentations had been identified in a previous vintage.