Aims: The studies of the production of exopolysaccharides by lactose-negative yeast and a yogurt starter cocultivated in a natural substrate containing lactose may be considered of interest because they reveal the possibilities for high-efficiency synthesis of blopolymers by mixed cultivation. Methods and Results: The mixed culture Rhodotorula rubra GED10 + (Streptococcus thermophilus 13a + Lactobacillus bulgaricus 2-11) was cultivated in cheese whey ultrafiltrate (WU) (44.0 g lactose l(-1)) at initial pH 6-0, 28degreesC, under intensive aeration (air-flow rate 1.0 1 l(-1) min(-1), agitation 220 rev min(-1)) in a MBR AG fermentor. The mixed culture manifested the highest activity for synthesis of exopolysaccharides (19.3 g l(-1)) and cell mass (21.0 g l(-1)) at the 84th hour. The yogurt starter synthesized neutral exopolysaccharides, while the mixed culture yeast + yogurt starter produced acidic exopolysaccharides containing uronic acid (6%). The neutral sugar composition was identified as mannose, glucose, galactose, xylose and arabinose. Mannose dominated in the polymer composition (83%) that was produced only by the yeast (97%). Conclusions: Lactose in the WU can be effectively utilized by a co-culture of lactose-negative yeast-yogurt starter for synthesis of exopolysaccharides. Significance and Impact of the Study: The present findings propose an alternative use of WU as a cost-effective carbohydrate substrate, and suggest that the lactose-negative yeast Rhodotorula rubra can have industrial application as producers of exopolysaccharides.