Carotenogenesis of the lactose-negative yeast Rhodotorula rubra GED5 was studied by cocultivation with Kluyveromyces lactis MP11 in whey ultrafiltrate (WU) (35, 50, and 70 g of lactose/L). Maximum yields of cell mass (24.3 g/L) and carotenoids (10.2 mg/L of culture fluid or 0.421 mug/g of dry cells) were obtained by growing the microbial association in WU (50 g of lactose/L) in a fermentor with an airflow rate of 0.8 L/(L(.)min), agitation of 220 rpm, and temperature of 30degreesC. The identified carotenoid pigments-beta-carotene, torulene, and torularhodin-reached maximum concentrations (133, 26.9, and 222.3 mug/g of dry cells, respectively) on d 5 for torulene and d 6 for beta-carotene and torularhodin.