Aims: Analysis of the diversity and distribution of wine yeasts isolated from organically and conventionally grown grapes, and during the subsequent fermentation with or without starter cultures in six different commercial wineries. Methods and Results: PCR-RFLP screening of isolates revealed the involvement of ten different species. Saccharomyces cerevisiae, scarcely isolated from grapes, was the dominant species during the latter phases of fermentation, identifying 108 different genotypes by means of SSR analysis. Species and strains' diversity and presence were strongly influenced by the farming system used to grow the grapes and the system of vinification. Conclusions: Organic farming management was more beneficial in terms of diversity and abundance than the conventional one. Induced fermentation generated a great replacement of native yeasts. Although winery-resident yeasts resulted to be predominant in the process, some noncommercial strains originally in the vineyard were found in final stages of the fermentation, confirming that autochthonous strains of S. cerevisiae are capable to conduct the fermentation process up to its end. Significance and Impact of the Study: The study of natural yeast communities from commercial vineyards and wineries is an important step towards the preservation of native genetic resources. Our results have special relevance because it is the first time that the real situation of the yeast ecology of alcoholic fermentation in commercial wineries belonging to the relevant wine-producing Appellation of Origin ` Vinos de Madrid' is shown.