Winery wastewaters are characterised by large seasonal fluctuations in volume and composition and are often discarded with little or no treatment. A rotating biological contactor (RBC) was used to investigate microorganisms associated with the biological treatment of winery wastewater. Extensive biofilms developed on the RBC discs and contained a number of yeast and bacterial species that displayed a dynamic population shift during the evaluation period. This suggested that the naturally occurring microorganisms were able to form a stable biofilm and also reduce the chemical oxygen demand (COD) of winery wastewater (on average 43% with a retention time of 1h). One of the yeast isolates, MEA 5, was able to reduce the COD of synthetic wastewater by 95% and 46% within 24h under aerated and non-aerated conditions, respectively. The yeast isolates could therefore play an important role in the degradation of organic compounds under aerobic conditions, such as those associated with an RBC.