Aims: To study the role of the indigenous yeast flora in traditional Irish cider fermentations.Methods and Results: Wallerstein laboratory nutrient agar supplemented with biotin, ferric ammonium citrate, calcium carbonate and ethanol was employed together with PCR-restriction fragment length polymorphism analysis of the region spanning the internal transcribed spacers (ITS1 and ITS2) and the 5.8S rRNA gene in the identification of indigenous yeasts at the species level, from traditional Irish cider fermentations. By combining the molecular approach and the presumptive media it was possible to distinguish between a large number of yeast species, and to track them within cider fermentations. The Irish cider fermentation process can be divided into three sequential phases based on the predominant yeast type present. Kloeckera/Hansentaspora uvarum type yeasts predominate in the initial 'fruit yeast phase'. Thereafter Saccharomyces cerevisiae type yeast dominate in the 'fermentation phase', where the alcoholic fermentation takes place. Finally the 'maturation phase' which follows, is dominated by Dekkera and Brettanomyces type yeasts. H. uvarum type yeast were found to have originated from the fruit. Brettanomyces type yeast could be traced back to the press house, and also to the fruit. The press house was identified as having high levels of S. cerevisiae type yeast. A strong link was noted between the temperature profile of the cider fermentations, which ranged from 22 to 35degreesC and the yeast strain population dynamics.Conclusions: Many different indigenous yeast species were identified. The mycology of Irish cider fermentations appears to be very similar to that which has previously been reported in the wine industry.Significance and Impact of the Study: This study has allowed us to gain a better understanding of the role of indigenous yeast species in 'Natural' Irish cider fermentations.