Seeking to understand the dynamics of the yeast population in a bioethanol fermentation process that uses cell recycling, the yeast inhabiting the fermentation tanks throughout the production season were monitored. A total of 26 yeasts were isolated from tanks in a Brazilian bioethanol distillery plant during six different periods of the season. These yeasts were evaluated with regard to fermentative capacity and all yeasts were qualified to be used for bioethanol production. Based on the numerical taxonomy, it was possible to say that they were all representative of Saccharomyces sensu stricto. A total of 10 different banding patterns were obtained from the 16 strains isolated. This work has shown that the yeast introduced at the beginning of the season was quickly replaced by one or more native yeast strains. It was also shown that the replacement of these strains is not always harmful to the process and isolating such yeasts found in the fermentation tanks could be an interesting strategy for new strain selection.