A dynamic fed-batch microcosm system is described which permits assessment of the progressive growth of yeasts through olive oil waste. We report on its application to measure the effects of the growth of yeast strains upon the chemical composition of "alpeorujo", the waste of a two-phase decanter system used for the extraction of olive oil. Six phenotypically distinct groups of yeasts were isolated. Three selected isolates were identified as being most closely related to Saccharomyces sp., Candida boidinii and Geotrichum candidum using biochemical tests and partial I SS rDNA gene sequence analysis. This is the first report of yeast growth on "alpeorujo" by the use of a fed-batch microcosm system, resulting in the change of the initial chemical composition of "alpeorujo" and in the decrease of the toxic substances such as phenols, (C) 2003 Elsevier Ltd. All rights reserved.