Aims: To evaluate the effect of yeast extract (YE) concentration, temperature and pH on growth and exopolysaccharide (EPS) production in a whey-based medium by Streptococcus thermophilus SY and to characterize the partially purified EPS. Methods and Results: Factorial experiments and empirical model building were used to optimize fermentation conditions and the chemical composition, average molecular weight (MW) and theological properties of aqueous dispersions of the EPS were determined. Exopolysaccharide production was growth associated and was higher (152 mg l(-1)) at pH 6.4 and 36degreesC with 4 g l(-1) YE. High performance size exclusion chromatography of the partially purified EPS showed two peaks, with a weight average MW of 2 X 106 and 5 X 104, respectively. The EPS was a heteropolysaccharide, with a glucose : galactose : rhamnose ratio of 2 : 4.5 : 1. Its water dispersions had a pseudoplastic behaviour and showed a higher viscosity of xanthan solutions. Significance and Impact of the Study: The fermentation conditions and some properties of an EPS produced by Strep. thermophilus, a dairy starter organism, were described.